Retro-focus type lens and projection-type display apparatus

ABSTRACT

A retro-focus-type lens is provided from a large conjugate side with a first lens group having a negative refracting power, a second lens group having a positive refracting power which is spaced away from the first lens group at an air interval D1, and a third lens group having a positive refracting power. The following conditions are satisfied when focal lengths of the first lens group, the second lens group, the third lens group, and a total lens system are assumed respectively as f1, f2, f3, and f: 
     1.8&lt;D1/f&lt;3.0 
     1.1&lt;f2/f3&lt;1.6 
     1.5&lt;|f1|/f&lt;2.3. 
     A projection lens constructed by inserting a mirror in a lens system of thus constructed retro-focus type lens is used in a projection-type display apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates a projection lens which allows production of a small size projection-type display apparatus, a projection-type display apparatus using the projection lens, and a projection-type display apparatus capable of changing the contrast and the luminance.

2. Description of Related Art

FIG. 1 is a diagram showing an optical system of a conventional projection-type display apparatus using liquid crystal panels, which apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 1-120192. In this figure, indicated at 1 is a light source including a lamp 120 and a reflecting mirror 130. The light source 1 emits an illumination beam 2. As a lamp 120 is used, for example, a white light source such as a metal halide lamp, a xenon lamp, and a halogen lamp. A reflecting surface of the reflecting mirror 130 is typically a paraboloid or spherical surface, and a luminescence center of the lamp 120 is positioned at a focus position of the paraboloid or a center of curvature of the spherical surface as well known. Along an emitting direction of the illumination beam 2 are arranged a dichroic mirror 14B which reflects a blue light while allowing green and red lights to transmit therethrough, a dichroic mirror 14G which reflects the green light while allowing the red light to transmit therethrough, and a mirror 11a for reflecting the light entirely. A mirror 11c for reflecting the light entirely is arranged in a reflecting direction of the dichroic mirror 14B, and a liquid crystal panel 3B is arranged in a reflecting direction of the mirror 11c. A liquid crystal panel 3G is arranged in a reflecting direction of the dichroic mirror 14G. A mirror 11b for reflecting the light entirely is arranged in a reflecting direction of the mirror 11a, and a liquid crystal panel 3R is arranged in a reflecting direction of the mirror 11b. A drive circuit for causing an image to be displayed on the liquid crystal panels 3R, 3G, 3B is unillustrated. A dichroic prism 15 which selectively reflects the red and blue lights and selectively allows the green light to transmit therethrough is arranged and surrounded by these liquid crystal panels 3R, 3G, 3B. A projection lens 4 is arranged in an emitting direction of the dichroic prism 15. The respective members constituting the optical system before the projection lens 4 are held in a casing 200, and the casing 200 and the projection lens 4 constitute a projector 300.

An operation of the projector 300 will be described next. The illumination beam 2 emitted from the light source 1 is separated into lights of three primary colors, namely red, green, and blue, by the dichroic mirrors 14B and 14G, and the respective color lights are reflected by the mirrors 11a, 11b, 11c and projected on the liquid crystal panels 3R, 3G, 3B for displaying monochromatic images corresponding to the respective primary colors. The lights modulated by the images formed on the liquid crystal panels 3R, 3G, 3B are combined into a single beam again by the dichroic prism 15. The combined single beam is converted into a projected light 110 by the projection lens 4 to be projected on a screen as described later. The projection lens 4 is required to correct various aberrations in order to obtain a satisfactory projected image, and includes a combination of a plurality of single lenses (not shown). The projected image on the screen is focused by driving a part of lens elements constituting the projection lens 4 or the entire projection lens 4 along an optical axis.

FIG. 2 is a diagram showing an optical system of another conventional projection-type display apparatus using the liquid crystal panels, which apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 1-157688. In this figure, indicated at 131 is a condenser lens provided at the front portion of a light source 1. In this example, the light source 1 includes a lamp 120, a reflecting mirror 130, and the condenser lens 131. Other construction is same as the aforementioned prior art (see FIG. 1), and no description will be given thereon by giving like reference numerals to like parts. Further, since the operation of this display apparatus is similar to the one of the aforementioned prior art, description thereof is omitted.

As a projection-type display apparatus, there have been conventionally known a projection-type display apparatus of the front system by which the projected light emitted from the projection lens is formed as an image on a reflection type screen, and a projection-type display apparatus of the rear system in which a projector is accommodated in a cabinet and the projected light is formed as an image on a translucent screen. FIG. 3 shows a construction of a projection-type display apparatus of the rear system disclosed in Japanese Utility Model Application Laid-Open No. 1-115778. In this figure, indicated at 170 is a cabinet for accommodating a projector 300 including a casing 200 and a projection lens 4. The projector 300 has a configuration similar to that of the projector 300 in FIG. 1 or 2. In the casing 200 are held an optical system (not shown) including from a light source 1 to a dichroic prism 15. On a side wall of the cabinet 170 is arranged a translucent screen 5. In the cabinet 170 are arranged bent mirrors 150 and 160 for introducing a projected light 110 emitted from the projection lens 4 to the screen 5.

An operation of the apparatus shown in FIG. 3 will be described next. The projected light 110 emitted from the projection lens 4 is formed as an enlarged image on the screen 5 after being reflected by the bent mirrors 150 and 160. The bent mirrors 150 and 160 are employed to bend an optical path extending from a leading end of the projection lens 4 to the screen 5 so as to accommodate the optical system in the cabinet 170 in a compact manner. A viewer 400 views the enlarged image from a direction opposite to the projector 300 relative to the screen 5.

In the projection-type display apparatus of the rear system, a portion 171 below the screen 5 is large in the cabinet 170 since the bent mirror 150 and the projector 300 are accommodated below the screen 5 as shown in FIG. 3. In view of this, it has been difficult to reduce the height H of the optical projection set. Thus, there has been adopted a method of projecting the projected light 110 directly behind the screen by omitting the bent mirrors 150 and 160 from the optical system so as to set the height H substantially equal to the level of the screen 5. In this case, the problem is such that the depth D of the set is increased to thereby increase an installation area of the cabinet 170.

In addition to the above, it has been effective to shorten a projection distance (length of a central beam from an emitting end of the projection lens 4 to the screen 5) in order to make the set small size. Accordingly, there has been adopted a method of widening an angular field of the projection lens 4. However, if a divergent angle of the projected light 110 is widened by widening the angular field of the projection lens 4, the large size bent mirror 150 is inevitably required. Thus, there has been a limit in reducing the height H of the set.

Conventionally, there has been a demand to view an image of the increased luminance since the projected image becomes relatively dark and hard to see when the luminance on the screen surface is high due to the external light. Conversely, when the luminance of the external light is low, there has been a demand to view an image by improving the contrast (particularly lowering the luminance of a black level) while suppressing the luminance of the projected image. However, the conventional projection-type display apparatuses are not provided with a function of automatically adjusting the luminance and the contrast according to the illuminated state of the place where the apparatus is installed.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a retro-focus type lens which can be applied to a small size projection-type display apparatus and in which a bent mirror can be built.

It is another object of the invention to provide a small size projection-type display apparatus having reduced depth and height.

It is still another object of the invention to provide a small size projection-type display apparatus which is substantially as large as a screen when viewed from the front and has a reduced depth.

It is further another object of the invention to provide a multi-vision projection-type display apparatus having a small depth in which a plurality of projectors for use in a small size projection-type display apparatus are arranged in rows and columns.

It is another object of the invention to provide a projection-type display apparatus capable of changing a contrast ratio and luminance of a projected image in the case where a device having a function of switching a state between a transparent state and a scattering state according to the brightness of the image is used as an image display device.

It is still another object of the invention to provide a projection-type display apparatus capable of automatically changing a contrast ratio and/or luminance of a projected image according to the brightness of an external light available where the apparatus is installed.

A retro-focus type lens according to the first invention comprises from a large conjugate side a first lens group having a negative refracting power, a second lens group having a positive refracting power which is spaced away from the first lens group at an air interval D1, and a third lens group having a positive refracting power, wherein the following conditions are satisfied when focal lengths of tile first lens group, the second lens group, the third lens group, and a total lens system are assumed respectively as f1, f2, f3, and f:

1.8<D1/f<3.0

1.1<f2/f3<1.6

1.5<|f1|/f<2.3.

In the first invention, mirror means for bending an optical path may be inserted between the first lens group and the second lens group. Also, in the first invention, diaphragm means may be arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side. Further, in the first invention, diaphragm means may be arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side, and an aperture diameter of the diaphragm means may be adjustable. Moreover, in the first invention, diaphragm means may be arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side; and motor means may be provided to cause an aperture diameter of the diaphragm means to vary. Furthermore, in the first invention, a lens element on the small conjugate side may be an aspherical lens out of those constituting the third lens group. Also, in the first invention, a lens element on the small conjugate side may be an aspherical lens out of those constituting the third liens group, and the following condition may be satisfied when focal lengths of the aspherical lens and the total lens system are assumed respectively as f4 and f: |f4|/f>10. Further, in the first invention, a lens element on the large conjugate side may be an aspherical lens out of those constituting the first lens group. Moreover, in the first invention, a lens element on the large conjugate side may be an aspherical lens out of those constituting the first lens group, and the following condition may be satisfied when focal lengths of the aspherical lens and the total lens system are assumed respectively as f5 and f: |f5|/f>100. Furthermore, in the first invention, a lens element on the small conjugate side out of those constituting the first lens group and a lens element constituting the second lens group may be both aspherical lenses. Also, in the first invention, an aberration correction may be carried out in a state where a parallel plane plate formed of plastic material in not less than 90% of its thickness is inserted between the third lens group and an image surface on a small conjugate side. Further, in the first invention, diaphragm means may be arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side, the second lens group may be moved along an optical axis to change the focal length of the total lens system, and the first lens group, the second lens group, and the third lens group may be integrally moved along the optical axis to change a projection magnification while compensating for a shift of the image surface. Moreover, in the first invention, diaphragm means may be arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side, the second lens group may be moved along the optical axis to change the focal length of the total lens system, and the projection magnification is changed while compensating for the shift of the image surface by integral movement of the first lens group, the second lens group, and the third lens group along the optical axis in combination with movement of the first lens group along the optical axis.

With the retro-focus type lens of the first invention, there can be obtained a projection lens having the following performances required for a liquid crystal projection-type display apparatus:

(A) A sufficient interval required to insert the mirror can be attained between the first lens group and the second lens group;

(B) A relatively long back focal length can be attained because of a retro-focus type construction;

(C) Since a satisfactory telecentric characteristic can be obtained despite a wide-angle lens, an off-axis beam becomes substantially parallel with the optical axis on the liquid crystal panel side, thereby eliminating an influence of incidence-angle dependency of a color-composition system; and

(D) A satisfactory peripheral illuminance ratio can be obtained despite the wide-angle lens.

Further, in the first invention, an angle of view is widened by introducing aspherical lenses as the lens element of the first lens group on the screen side or the lens element of the third lens group on the image surface side, thereby correcting the off-axis aberration, particularly the distortion and the astigmatism, satisfactorily. Also, by introducing aspherical lenses as the lens element of the first lens group on the image surface side and the lens element of the second lens group, the on-axis aberration and the off-axis aberration can be corrected in highly well-balanced manner. In addition, since an effective diameter of the lens is relatively small, the aspherical surfaces can be formed easily. If the parallel plane plate formed of plastic material in most of its thickness is inserted between the third lens group and the image surface so as to correct the aberration supposing that the dichroic prism for color composition is formed of plastic material such as PMMA, the projection-type display apparatus using this retro-focus type lens can be produced light weighted and inexpensively. Moreover, the focal length of the total lens system is changed by making the second lens group movable along the optical axis. The shift of the image surface resulting from the varied focal length is compensated for by the integral movement of the first, second, and third lens groups along the optical axis, and thereby the projection lens serves as a zoom lens and is capable of adjusting the size of the projected image finely.

In a projection-type display apparatus according to the second invention, a first mirror is built in a projection lens, a portion of the projection means before the first mirror is mounted at a left side or a right side from a center of the screen when viewed from the front of the apparatus, and the projected light from the projection lens is reflected in a substantially vertical direction by the first mirror and then bent in a substantially horizontal direction by a second mirror so as to be incident upon a screen. Also, in the second invention, when the optical path is bent in the vertical direction by the first mirror, a direction of the image display device corresponding to the horizontal direction of the screen may be caused to coincide with a direction of a center ray of an exiting light from the projection lens according to rotation of a display direction of the image display device to be displayed as the horizontal direction on the screen.

According to the second invention, unlike the conventional apparatuses, since most of the projection means is accommodated at the right side or left side when viewed from the front center of the screen, and the optical path is bent on the plane when viewed from the front of the apparatus and on the plane when viewed from a lateral direction of the apparatus, the height and the depth of the apparatus can be reduced at the same time. In addition, since the first mirror is built in the projection lens, the first mirror is not to become large in size even if a wide-angle projection lens is used.

A projection-type display apparatus according to the third invention comprises first and second mirrors similar to those of the second invention. A casing for accommodating an optical system of projection means before a projection lens is arranged on a left side or right side from the center of a screen when viewed from the front of the apparatus. An optical system of the projection means before the first mirror is arranged on a plane perpendicular to a center ray of an exiting light from the projection lens, and an angle defined between a bottom portion of the casing provided substantially in parallel with the plane where the optical system is arranged and a bottom portion of a cabinet for accommodating the casing is set within a specified angle range. In the third invention, the angle defined by these bottom portions is set 20° or smaller.

According to the third invention, the casing for holding an optical system of the projection means before the projection lens is accommodated at the right side or left side from the front center of the screen when viewed from the front center, the optical system is held on the plane parallel with the bottom surface of the projection means in the casing, and the bottom surface of the casing can be arranged in suitable angular relation with the bottom surface of the cabinet for holding the entire apparatus. Accordingly, the height and the depth of the apparatus can be reduced at the same time. In addition, since the first mirror is built in the projection lens, the first mirror is not to become large in size even if a wide-angle projection

A projection-type display apparatus according to the fourth invention comprises a mirror provided in a projection lens of projection means, the mirror being adapted to bend an optical path. A center ray of a projected light exiting from the projection lens is bent in a substantially vertical direction by the mirror so as to be incident upon a translucent screen. An optical system of the projection means before the mirror is arranged on a plane substantially perpendicular to the center ray of an exiting light from the projection lens. A bottom portion of the casing for holding the optical system, provided substantially in parallel with the plane where the optical system is arranged, is substantially parallel with the screen. In the fourth invention, it may be appropriate to array a plurality of projection means in rows and columns and to form a unit image by a projected light of each projection means, so that adjoining unit images are arrayed close to one another.

According to the fourth invention, the projected light to be incident upon the screen is bent in the horizontal direction by the mirror built in the projection lens, and the optical system before the projection lens is arranged on the plane substantially parallel with the screen. Accordingly, the depth of the apparatus can be reduced while making the front dimensions of the apparatus substantially equal to the dimensions of the screen. In addition, since the mirror is built in the projection lens, the mirror is not to become large in size even if a wide-angle projection lens is used unlike a case where the mirror is arranged after the lens. Further according to the fourth invention, since the front dimensions of the apparatus can be made substantially equal to the dimensions of the screen as described above, there can be realized a multi-vision projection-type display apparatus having a small depth and small joining lines between adjoining unit images by arranging a plurality of projection means in rows and columns and arraying the unit images formed by the projected lights from the respective projection means closely in rows and columns.

In the second, third, or fourth invention, a dichroic prism formed of plastic material for combining lights of three primary colors may be arranged before the projection lens.

If the dichroic prism for color composition is formed of plastic material such as PMMA, the apparatus can be produced light weighted and inexpensively compared to the one including the conventional dichroic prism formed of known glass material.

Further, in the second, third, or fourth invention, a zoom lens capable of adjusting the projection magnification may be used as the projection lens.

By using the zoom lens capable of finely adjusting the projection magnification, the dimensional ratio of a periodic structure of the screen to that of the projected image on the liquid crystal panel is made variable. Accordingly, the moire superimposed on the projected image can be adjusted to a low level of visibility where the moire practically causes no problem. Further, by using the zoom lens as the projection lens of each projection means constituting the multivision projection-type display apparatus, the projection magnification of the respective projection means can be adjusted at the same value even if there is some difference in the projection magnification between unit images due to a manufacturing error of lenses and an arrangement error of optical parts.

A projection-type display apparatus according to the fifth invention includes an image display device provided with such a function that a state thereof is switched between a scattering state and a transparent state according to the brightness of a displayed image, and diaphragm means for restricting a recipient angle of a beam exiting from the image display device, an aperture diameter thereof being controllable.

According to the fifth invention, since the diaphragm means whose aperture diameter is changeable is provided within the projection lens and the recipient angle of the exiting light from the image display device in which the scattering / transparent states are switched is changeable, the contrast ratio and the luminance of the projected image can be adjusted in accordance with the desire of viewers, and new functions which cannot be found in the conventional projection-type display apparatuses can be added.

A projection-type display apparatus according to the sixth invention detects an illuminated state of a place where the apparatus is installed, and changes an aperture diameter of the diaphragm means for restricting a recipient angle of a beam exiting from an image display device and/or a drive power of a lamp for illuminating the image display device according to the detected illuminated state. In the sixth invention, the image display device may be provided with such a function that a state thereof is switched between a scattering state and a transparent state according to the brightness of a displayed image.

According to the sixth invention, since the aperture diameter of the diaphragm means within the projection lens is controlled according to the brightness of the external light, the recipient angle of the beam exiting from the image display device having the states switchable between the scattering state and the transparent state is made variable. Thus, the contrast ratio and the luminance of the projected image can be automatically adjusted to satisfactory levels when the external light changes. Further, since the drive power of the lamp for illuminating the image display device is variable according to the brightness of the external light, the luminance and the contrast ratio of the projected image can be adjusted automatically independently when the external light changes. Thus, new functions which cannot be found in the conventional projection-type display apparatuses can be added to the display apparatus according to the invention.

The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a construction diagram of a projector for use in a liquid crystal projection-type display apparatus of the prior art;

FIG. 2 is a construction diagram of a projector for use in a liquid crystal projection-type display apparatus of the prior art;

FIG. 3 is a construction diagram of a liquid crystal projection-type display apparatus of the prior art;

FIG. 4 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 1);

FIG. 5 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 1);

FIG. 6 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 2);

FIG. 7 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 3);

FIG. 8 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 3);

FIG. 9 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 4);

FIG. 10 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 4);

FIG. 11 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 5);

FIG. 12 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 5);

FIG. 13 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 6);

FIG. 14 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 6);

FIG. 15 is a sectional view showing a retro-Focus type lens according to the first invention (embodiment 7);

FIG. 16 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 7);

FIG. 17 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 8);

FIG. 18 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 8);

FIG. 19 is a sectional view showing a retro-focus type lens according to the first invention (embodiments 9 to 12, to 20);

FIG. 20 is a sectional view showing a retro-focus type lens according to the first invention (embodiments 9 to 12, to 20);

FIG. 21 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 13);

FIG. 22 is a sectional view showing a retro-focus type lens according to the first invention (embodiments 21, 22);

FIG. 23 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 23);

FIG. 24 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 24);

FIG. 25 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 24);

FIG. 26 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 25);

FIG. 27 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 25);

FIG. 28 is a sectional view showing a retro-focus type lens according to the first invention (embodiment 25);

FIG. 29(a), 29(b) and 29(c) are sectional views showing a retro-focus type lens according to the first invention (embodiment 25);

FIGS. 30(a), 30(b), and 30(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 1);

FIGS. 31(a), 31(b), and 31(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 2);

FIGS. 32(a), 32(b), and 32(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 3);

FIGS. 33(a), 33(b), and 33(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 4);

FIGS. 34(a) , 34(b), and 34(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 5);

FIGS. 35(a), 35(b), and 35(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 6);

FIGS. 36(a), 36(b), and 36(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 7);

FIGS. 37(a), 37(b), and 37(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 8);

FIGS. 38(a), 38(b), and 38(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 9);

FIGS. 39(a), 39(b), and 39(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 10);

FIGS. 40(a), 40(b), and 40(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 11);

FIGS. 41(a), 41(b), and 41(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 12);

FIGS. 42(a), 42(b), and 42(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 13);

FIGS. 43(a), 43(b), and 43(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 14);

FIGS. 44(a), 44(b), and 44(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 15);

FIGS. 45(a), 45(b), and 45(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 16);

FIGS. 46(a), 46(b), and 46(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 17);

FIGS. 47(a), 47(b), and 47(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 18);

FIGS. 48(a), 48(b), and 48(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 19);

FIGS. 49(a), 49(b), and 49(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 20);

FIGS. 50(a), 50(b), and 50(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 21);

FIGS. 51(a), 51(b), and 51(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 22);

FIGS. 52(a), 52(b), and 52(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 23);

FIGS. 53(a), 53(b), and 53(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 24);

FIGS. 54(a), 54(b), and 54(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 25);

FIGS. 55(a), 55(b), and 55(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 25);

FIGS. 56(a), 56(b), and 56(c) are diagrams showing various aberrations of the retro-focus type lens according to the first invention (embodiment 25);

FIG. 57(a) and 57(b) are overall construction diagrams of a projection-type display apparatus according to the second invention, the third invention, and the fifth invention;

FIG. 58(a) and 58(b) are construction diagrams of a projector incorporated in the projection-type display apparatus according to the second invention, the third invention, and the fifth invention;

FIG. 59(a), 59(b) and 59(c) are overall construction diagrams of a projection-type display apparatus according to the fourth invention and the sixth invention;

FIG. 60(a) and 60(b) are construction diagrams of a projector incorporated in the projection-type display apparatus according to the fourth invention and the sixth invention;

FIG. 61 is an overall construction diagram of a multivision projection-type display apparatus according to the fourth invention;

FIGS. 62(a) , 62(b) , 62(c), and 62(d) are diagrams showing effects of the fifth invention in the case where an aperture diameter of a projection lens is made variable; and

FIGS. 63(a) , 63(b) , 63(c) , and 63(d) are diagrams showing effects of the sixth invention in the case where an aperture diameter of a projection liens is made variable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the invention will be described in detail with reference to the drawings illustrating embodiments thereof.

First Invention

A wide-angle projection lens is necessary in order to realize a small size liquid crystal projection-type display apparatus of the rear system. A wide-angle lens for use in a single lens reflex camera has been conventionally known as a lens to obtain a wide-angle image. However, as a projection lens for use in the liquid crystal projection-type display apparatus according to the invention, the following specifications are additionally considered for the wide-angle lens for use in the single lens reflex camera.

(A) The wide-angle lens is so constructed as to attain a sufficient air interval in a lens system to insert a bent mirror therein.

(B) A wide-angle lens is capable of attaining a long back focal length in order to insert a dichroic prism for color composition.

(C) A color shade is generated when a chief ray of an off-axis beam is inclined relative to the dichroic prism. In the case where an illumination beam is a parallel beam (telecentric illumination), an off-axis F-number is increased and a peripheral light amount is reduced equivalently when the chief ray of the off-axis beam on the liquid crystal panel side is inclined. Thus, as the projection lens, the chief ray of the off-axis beam on the liquid crystal panel side is in parallel with an optical axis of the lens (telecentric characteristic).

(D) Despite the wide-angle lens, a sufficient peripheral illuminance ratio can be attained.

The projection lens according to the invention satisfies the above requirements and provides a lens construction having a sufficient resolving power, and further provides a zoom lens construction capable of reducing the moire of a projected image and finely adjusting an image size by minutely changing a projection magnification.

Hereafter, the projection lens according to the invention will be described with reference to the drawings. FIGS. 4 to 29 are sectional views of lenses corresponding to representative constructions out of numerical embodiments of the invention to be described later. Though unillustrated, it is assumed that a screen is provided at the left side of FIGS. 4, 6, 7, 9, 11, 13, 15, 17, 19, 21, 22, 23, 24, 26, 27, and 28 and at an upper side of FIGS. 5, 8, 10, 12, 14, 16, 18, 20, 25, and 29. Indicated at G1 is a first lens group located on the screen side (large conjugate side) and having a negative refracting power, at G2 a second lens group having a positive refracting power, at G3 a third lens group located at the side of the liquid crystal panel 3 (small conjugate side) and having the positive refracting power. The second lens group G2 has a diaphragm AST in the vicinity of the side of the liquid crystal panel 3 and a focal point of the third lens group G3 on the screen side. Further, between the third lens group G3 and the liquid crystal panel 3 is provided a parallel plane plate L9 (L9 and L10 in FIG. 21) having an optical width of the width of the dichroic prism 15 plus the width of a cover glass (not shown) of the liquid crystal panel 3 in FIG. 1 or 2 showing the prior art.

FIGS. 4 and 5 show an embodiment 1. FIG. 5 shows a state where a bent mirror M inclined at 45° is inserted in the same lens system as in FIG. 4. FIG. 6 is a sectional view of a lens of an embodiment 2; FIGS. 7 and 8 are sectional views of a lens of an embodiment 3; FIGS. 9 and 10 are sectional views of a lens of an embodiment 4; FIGS. 11 and 12 are sectional views of a lens of an embodiment 5; FIGS. 13 and 14 are sectional view of a lens of an embodiment 6; FIGS. 15 and 16 are sectional views of a lens of an embodiment 7; FIGS. 17 and 18 are sectional views of a lens of an embodiment 8; FIGS. 19 and 20 are sectional views of a lens of embodiments 9 to 12, 14 to 20; FIG. 21 is a sectional views of a lens of an embodiment 12, FIG. 22 is a sectional view of a lens of embodiments 21, 22; FIG. 23 is a sectional view of a lens of an embodiment 23; FIGS. 24 and 25 are sectional views of a lens an embodiment 24; and FIGS. 26 to 29 are sectional views of a lens of an embodiment 25.

Construction of Lens Groups

(Embodiments 1 to 8) (See FIGS. 4, 6, 7, 9, 11, 13, 15, 17)

The first lens group G1 includes a first lens element L1 having the positive refracting power and a convex surface on the large conjugate side, a second lens element L2 having the negative refracting power and directing a strong concave surface to the small conjugate side, and a third lens element L3 having the negative refracting power and directing a strong concave surface to the small conjugate side. The second lens group G2 includes a fourth lens element L4 having the positive refracting power, and the diaphragm AST is provided in the vicinity of the small conjugate side and the focal point of the third lens group G3 on the large conjugate side. The third lens group G3 includes a fifth lens element L5 having the negative refracting power and a concave surface on the small conjugate side, a sixth lens element L6 having the positive refracting power and a convex surface on the large conjugate side which is joined with the lens element L5, seventh and eighth lens elements L5, L8 having the positive refracting power.

(Embodiments 9 to 20) (See FIGS. 19 and 21)

The construction of the First lens group G1 and the second lens group G2 are same as the embodiments 1 to 8. The third lens group G3 includes a fifth lens element L5 having the negative refracting power and a concave surface on the small conjugate side, a sixth lens element L6 having the positive refracting power and a convex surface on the large conjugate side which is joined with the lens element L5, a seventh lens element L7 having the positive refracting power, and an aspherical eighth lens element L8 whose center portion has the weak refracting power and whose peripheral portion has the strong negative refracting power.

(Embodiments 21, 22) (See FIG. 22)

Compared to the construction of the embodiments 1 to 20, the construction of the first lens group G1 includes additionally an aspherical lens element L0 whose center portion has the weak refracting power on the large conjugate side, and whose peripheral portion has the strong positive refracting power. The construction of the second lens group G2 is same as in the embodiments 1 to 20. Similar to the embodiments 1 to 20, the third lens group G8 includes fifth and sixth lens elements L5, L6, and spherical lens elements L7, L8 having the positive refracting power.

(Embodiment 23) (See FIG. 23)

The construction of the first lens group G1 is same as in the embodiments 1 to 20 except that a third lens element L3 is an aspherical lens. The construction of the second lens group G2 is same as in the embodiments 1 to 20 except that a fourth lens element L4 is an aspherical lens. The construction of the third lens group G3 is same as in the embodiments 21, 22.

(Embodiment 24) (See FIG. 24)

The construction of the first lens group G1 and the second lens group G2 is same as in the embodiments 1 to 20. The construction of the third lens group G3 is same as in the embodiments 21, 22.

(Embodiment 25) (See FIGS. 26 to 28)

The construction of the first to third lens groups G1 to G3 are same as in the embodiments 1 to 20. However, in this embodiment, a lens is a zoom lens capable of finely adjusting the projection magnification. Thus, the focal length of the total lens system is made variable by moving a lens element L4 of the second lens group G2 along the optical axis, and movement of the image surface caused by the varied focal length is compensated for by movement of the first to third lens groups G1 to G3 along the optical axis.

As seen from the above, in the respective embodiments, the first lens group G1 having the negative refracting power and the second lens group G2 and the third lens group G3 having the positive refracting power are combined to construct the retro-focus type lens, and there is attained such a sufficiently long back focal length to insert the thick parallel plane plate L9 (L9, L10 in the embodiment 13 shown in FIG. 21) after the third lens group G3 and to provide proper air intervals before and after the parallel plane plate L9. Further, the diaphragm AST is arranged in the vicinity of the focal point of the third lens group G3 on the screen side (on the large conjugate side), thereby realizing a telecentric construction in which an off-axis chief ray becomes substantially parallel with the optical axis on the side of the liquid crystal panel 3 (on the small conjugate side). Moreover, a large air interval is attained between the first lens group G1 and the second lens group G2, thereby allowing the bent mirror M to be inserted in that air interval.

In order to achieve the aforementioned specifications (A) to (D) of the projection lens, the lens system according to the invention satisfies the following conditional equations on the assumption that the focal lengths of the first, second, third lens groups G1, G2, G3, and the total lens system are f1, f2, f3, f, and the air interval between the first lens group G1 and the second lens group G2 is D1.

1.8<D1/f<3.0 (1)

1.1<f2/f3<1.6 (2)

1.5<|f1|/f<2.3 (3)

Description will be given on the meanings of upper and lower limit values of the above conditional equations. First of all, in the equation (1) for determining a ratio of the air interval between the first lens group G1 and the second lens group G2 to the focal length of the total lens system, if the ratio is in excess of the upper limit value of the equation (1), it is advantageous in inserting the mirror M. However, this in turn makes an outside diameter of the first lens group G1 larger and makes the length of the lens system unnecessarily long. Conversely, if the ratio is below the lower limit value of the equation (1), it becomes difficult to insert the mirror M. Further, in order to satisfy a required projection angle, the negative refracting power of the first lens group G1 becomes too strong, thereby making it difficult to correct an oblique aberration.

In the condition of the equation (2) for determining a ratio of the focal length of the second lens group G2 to that of the third lens group G3, if the ratio is in excess of the upper limit value of the equation (2), a positive refracting-power sharing of the third lens group G3 becomes too strong compared to the second lens group G2. Thus, if the off-axis chief ray is made substantially parallel with the optical axis on the small conjugate side, a large distortion is generated in the shape of a barrel off the optical axis when viewed at the small conjugate side, and it is difficult to correct that distortion. Conversely, if the ratio is below the lower limit value of the equation (2), a positive refracting-power sharing of the third lens group G3 becomes too weak compared to the second lens group G2. Accordingly, the construction of the retro-focus type lens becomes weak, thereby making it difficult to attain the long back focal length.

The equation (3) determines a ratio of the focal length of the first lens group G1 to that of the total lens system. If the ratio is in excess of the upper limit value of the equation (3), a negative refracting-power sharing of the first lens group G1 becomes too weak. Accordingly, the construction of the retro-focus type lens becomes weak, thereby making it difficult to attain the long back focal length. In order to attain the refracting power of the first lens group G1 and the long back focal length in a state where the ratio is in excess of the upper limit value of the equation (3), it is necessary to space the first lens group G1 and the second lens group G2 apart by an unnecessary long distance. This is not preferable since the entire length of the lens system is increased and the outside diameter of the first lens group G1 becomes larger accordingly. Conversely, if the ratio is below the lower limit value of the equation (3), the negative refracting power of the first lens group G1 becomes too strong, with the result that a large astigmatism is generated off the optical axis, thereby making it difficult to correct the astigmatism.

There will be shown numerical embodiments according to the invention. Meanings of symbols used in TABLE-1 to TABLE-25 showing the numerical embodiments are as follows. It will be appreciated that the focal lengths and the magnifications are values on an e line (546.1 nm).

f : Focal length of the entire projection lens system

θ: projection half-angle (on the large conjugate side)

F : effective F-number (on the small conjugate side) at a standard projection magnification

β: standard projection magnification

f1: focal length of the first lens group G1

f2: focal length of the second lens group G2

f3: focal length of the third lens group G3

f4: focal length of the aspherical lens L8 (Embodiments 9 to 20, 25)

f5: focal length of the aspherical lens L0 (Embodiments 21, 22)

D1: air interval between the first lens group G1 and the second lens group G2

m : surface number sequentially counted from the screen side

ri: radius of curvature of the i-th lens surface counted from the screen side

di: thickness of the i-th lens element and air interval counted from the screen side

ni: refractive index of the i-th lens element counted from the screen side at a wavelength of 546.1 nm (on the e line) (TABLE-1 to TABLE-8; Embodiments 1 to 8)

hi: refractive index of the i-th lens element counted from the screen side at a wavelength of 587.6 nm (on a d line) (TABLE-9 to TABLE-25; Embodiments 9 to 25)

ν: Abbe number of the i-th lens element counted from the screen side

AST: aperture plane

Further, it is understood that the shape of the aspherical surface indicated at * in TABLES-9 to 25 is expressed by the following equations (4), (5) in a rectangular coordinate system (X, Y, Z) wherein a center of the surface is set as an origin and the optical axis is set as a Z-axis, when r denotes a center radius of curvature, K denotes a conic constant, A4, A6, A8, and A10 denote fourth, sixth, eighth, and tenth order polynomial deformation terms. ##EQU1##

First Invention, Embodiment 1

FIG. 4 shows an embodiment 1. Numerical data of this embodiment are shown in TABLE-1. FIG. 5 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 4 and the light beam is caused to pass the lens system. In FIG. 5, a distance between a center (C1) of the last surface of the first lens group G1 and a center (C2) of the mirror M is set at 44.06 min. The diaphragm AST is arranged in close contact with the small conjugate side surface of lens element L4 constituting the second lens group G2. It can be seen from FIG. 5 that a sufficient air interval is attained to insert the mirror M between the first lens group G1 and the second lens group G2. There was obtained the peripheral illuminance ratio of 84.6% at a maximum projection half-angle (41.60°) because of effects of the off-axis pupil aberration. Generally, the peripheral illuminance ratio of 30% is required in the projection-type display apparatus. The above value is sufficiently high compared to this value. Further, the oblique angle of the chief ray was 0.3° or smaller. It can be also seen from FIG. 5 that the oblique angle of the off-axis beam with respect to the optical axis becomes smaller in an interior of the lens system than on the screen side (on the large conjugate side). Accordingly, the mirror M is allowed to be smaller sized than a case where it is arranged outside the projection lens (on the screen side).

                  TABLE 1                                                          ______________________________________                                         (EMBODIMENT 1)                                                                 f = 24.00  θ =41.60° F = 4.5  β = 23.5                       f1 = -41.35  f2 = 78.82  f3 = 64.78  D1 = 65.61                                D1/f = 2.73  f2/f3 = 1.22  |f1| /f = 1.72                    m       ri        di       ni    νi                                         ______________________________________                                         1       139.17585 8.90     1.69980                                                                              55.5                                          2       1401.53292                                                                               0.10                                                         3       132.10005 3.00     1.61686                                                                              37.0                                          4       32.25716  10.94                                                        5       164.89057 3.00     1.64129                                                                              55.5                                          6       35.81876  65.61                                                        7       134.01990 4.10     1.81264                                                                              25.5                                          8       -120.99001                                                                               47.31                 AST                                    9       -104.18790                                                                               2.57     1.81264                                                                              25.5                                          10      40.63287  13.71    1.73234                                                                              54.7                                          11      -70.25245 0.10                                                         12      172.28250 4.96     1.69980                                                                              55.5                                          13      -275.26863                                                                               0.10                                                         14      75.80423  7.84     1.69980                                                                              55.5                                          15      219.60430 5.00                                                         16      INF       51.00    1.51872                                                                              64.2                                          17      INF                                                                    ______________________________________                                    

First Invention, Embodiment 2

FIG. 6 shows an embodiment 2. Numerical data of this embodiment are shown in TABLE-2. In this embodiment, a spacing d6 (=D1) between the first lens group G1 and the second lens group G2 is set at 68.93 mm, thereby providing an air interval larger than the one in the embodiment 1 by 3.32 mm. Accordingly, there is no problem in inserting the mirror M.

                  TABLE 2                                                          ______________________________________                                         (EMBODIMENT 2)                                                                 f = 24.00  θ = 41.56° F = 4.5  β= 23.5                       f1 = -42.27  f2 = 79.73  f3 = 66.10  D1 = 68.93                                D1/f = 2.87  f2/f3 = 1.21  |f1| /f = 1.76                    m       ri        di       ni    νi                                         ______________________________________                                         1       138.07949 8.90     1.69980                                                                              55.5                                          2       1129.63736                                                                               0.10                                                         3       117.39695 2.01     1.61686                                                                              37.0                                          4       32.01591  11.58                                                        5       165.64017 2.01     1.64129                                                                              55.5                                          6       36.43719  68.93                                                        7       145.52849 4.10     1.81264                                                                              25.5                                          8       -115.30880                                                                               48.14                 AST                                    9       -101.53291                                                                               2.57     1.81264                                                                              25.5                                          10      41.07525  13.71    1.73234                                                                              54.7                                          11      -70.47261 0.10                                                         12      142.18918 4.96     1.69980                                                                              55.5                                          13      -440.96858                                                                               0.10                                                         14      79.84551  7.84     1.69980                                                                              55.5                                          15      259.57190 5.00                                                         16      INF       51.00    1.51872                                                                              64.2                                          17      INF                                                                    ______________________________________                                    

First Invention, Embodiment 3

FIG. 7 shows an embodiment 3. Numerical data of this embodiment are shown in TABLE-3. In this embodiment, the spacing d6 (=D1) between the first lens group G1 and the second lens group G2 is set at 53.83 mm, thereby providing the air interval smaller than the one in the embodiment 1 by 11.78 mm. FIG. 8 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 7 and the light beam is caused to pass the lens system. In FIG. 8, the distance between the center (C1) of the last surface of the first lens group G1 and the center (C2) of the mirror M is set at 34.6 mm. The diaphragm AST is arranged in close contact with the small conjugate side surface of lens element L4 constituting the second lens group G2. It can be seen from FIG. 8 that the mirror M can be easily inserted between the first lens group G1 and the second lens group G2.

                  TABLE 3                                                          ______________________________________                                         (EMBODIMENT 3)                                                                 f = 25.01  θ = 40.64°  F = 4.5  β = 23.5                     f1 = -39.74  f2 = 73.44  f3 = 61.20  D1 = 53.83                                D1/f = 2.15  f2/f3 = 1.20  |f1| /f = 1.59                    m       ri        di       ni    νi                                         ______________________________________                                         1       146.47790 8.90     1.69980                                                                              55.5                                          2       -9528.73000                                                                              0.10                                                         3       166.98639 3.00     1.61686                                                                              37.0                                          4       29.32481  6.68                                                         5       77.99256  3.00     1.64129                                                                              55.5                                          6       30.44297  53.83                                                        7       442.15849 4.10     1.81264                                                                              25.5                                          8       -68.70715 45.61                 AST                                    9       -101.08267                                                                               2.57     1.81264                                                                              25.5                                          10      40.44856  13.71    1.73234                                                                              54.7                                          11      -61.07050 0.10                                                         12      409.67064 4.96     1.69980                                                                              55.5                                          13      -163.35741                                                                               0.10                                                         14      95.94786  7.84     1.69980                                                                              55.5                                          15      688.57440 5.00                                                         16      INF       51.00    1.51872                                                                              64.2                                          17      INF                                                                    ______________________________________                                    

First Invention, Embodiment 4

FIG. 9 shows an embodiment 4. Numerical data of this embodiment are shown in TABLE-4. In this embodiment, the spacing d6 (=D1) between the first lens group G1 and the second lens group G2 is set at 47.65 mm, thereby providing the air interval smaller than the one in the embodiment 1 by 17.96 mm. FIG. 10 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 9 and the light beam is caused to pass the lens system. In FIG. 10, the distance between the center (C1) of the last surface of the first lens group G1 and the center (C2) of the mirror M is set at 28.5 mm. The diaphragm AST is arranged in close contact with the small conjugate side surface of lens element L4 constituting the second lens group G2. It can be seen from FIG. 10 that the mirror M can be mounted between the first lens group G1 and the second lens group G2 in a compact manner.

                  TABLE 4                                                          ______________________________________                                         (EMBODIMENT 4)                                                                 f = 23.83  θ = 41.94°  F = 4.5  β = 23.5                     f1 = -38.88  f2 = 74.83  f3 = 60.55  D1 = 47.65                                D1/f = 2.00  f2/f3 = 1.24  |f1| /f = 1.63                    m       ri        di       ni    νi                                         ______________________________________                                         1       142.84549 8.90     1.69980                                                                              55.5                                          2       2664.39249                                                                               0.10                                                         3       142.87467 3.00     1.61686                                                                              37.0                                          4       30.66992  7.58                                                         5       92.27039  3.00     1.64129                                                                              55.5                                          6       29.69118  47.65                                                        7       555.51124 21.91    1.81264                                                                              25.5                                          8       -67.07983 44.63                 AST                                    9       -100.21012                                                                               2.57     1.81264                                                                              25.5                                          10      42.13455  13.71    1.73234                                                                              54.7                                          11      -63.35782 0.10                                                         12      410.78378 4.96     1.69980                                                                              55.5                                          13      -132.22409                                                                               0.10                                                         14      118.11951 7.84     1.69980                                                                              55.5                                          15      INF       5.00                                                         16      INF       51.00    1.51872                                                                              64.2                                          17      INF                                                                    ______________________________________                                    

First Invention, Embodiment 5

FIG. 11 shows an embodiment 5. Numerical data of this embodiment are shown in TABLE-5. FIG. 12 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 11 and the light beam is caused to pass the lens system. In FIG. 12, the distance between a center (C1) of the last surface of the first lens group G1 and a center (C2) of the mirror M is set at 37.00 mm. The diaphragm AST is arranged behind the small conjugate side surface of lens element L4 constituting the second lens group G2 away therefrom by 1 mm (the ninth surface in TABLE-5). It can be seen from FIG. 12 that a sufficient air interval is attained to insert the mirror M between the first lens group G1 and the second lens group G2. There was obtained the peripheral luminance ratio of 81% at a maximum half-angle of view (43.30°) because of effects of the off-axis pupil aberration. Generally, the peripheral luminance ratio of 30% is required in the projection-type display apparatus. The above value is sufficiently high compared to this value. Further, the oblique angle of the chief ray was 0.6° or smaller. It can be also seen from FIG. 12 that the oblique angle of the off-axis beam with respect to the optical axis becomes smaller than tile outgoing beam does on the screen side (on the large conjugate side) in an interior of the lens system. Accordingly, the mirror M is allowed to have a smaller size than a case where it is arranged outside the projection lens (on the screen side).

                  TABLE 5                                                          ______________________________________                                         (EMBODIMENT 5)                                                                 f = 24.00  θ = 43.30°  F = 4.5  β = 23.5                     f1 = -42.27  f2 = 73.40  f3 = 58.09  D1 = 55.45                                D1/f = 2.31  f2/f3 = 1.26  |f1| /f = 1.76                    m       ri        di       ni    νi                                         ______________________________________                                         1       134.00000 12.00    1.69980                                                                              55.5                                          2       852.40000 0.20                                                         3       130.40000 3.00     1.61686                                                                              37.0                                          4       33.75800  11.25                                                        5       122.00000 3.00     1.64129                                                                              55.5                                          6       32.15000  55.45                                                        7       148.00000 10.00    1.81264                                                                              25.5                                          8       -96.89200 1.00                                                         9       INF       35.85                 AST                                    10      -73.90000 3.00     1.81264                                                                              25.5                                          11      39.40000  18.00    1.73234                                                                              54.7                                          12      -62.25000 0.20                                                         13      297.80000 6.05     1.69980                                                                              55.5                                          14      -165.07000                                                                               0.20                                                         15      87.20200  7.60     1.69980                                                                              55.5                                          16      INF       5.00                                                         17      INF       51.00    1.51872                                                                              64.2                                          18      INF                                                                    ______________________________________                                    

First Invention, Embodiment 6

FIG. 13 shows an embodiment 6. Numerical data of this embodiment are shown in TABLE-6. FIG. 14 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 13 and the light beam is caused to pass the lens system. In FIG. 14, the distance between the center (C1) of the last surface of the first lens group G1 and the center (C2) of the mirror M is set at 37.00 mm. The diaphragm AST is arranged behind the small conjugate side surface of lens element L4 constituting the second lens group G2 away therefrom by 1 mm (the ninth surface in TABLE-6). It can be seen front FIG. 14 that a sufficient air interval is provided to insert the mirror M between the first lens group G1 and the second lens group G2.

                  TABLE 6                                                          ______________________________________                                         (EMBODIMENT 6)                                                                 f = 24.00  θ = 43.15°  F = 4.5  β = 23.5                     f1 = -42.38  f2 = 73.60  f3 = 58.04  D1 = 55.49                                D1/f = 2.31  f2/f3 = 1.27  |f1| /f = 1.77                    m       ri        di       ni    νi                                         ______________________________________                                         1       133.71183 12.00    1.69980                                                                              55.5                                          2       897.73287 0.20                                                         3       131.74787 3.00     1.61686                                                                              37.0                                          4       33.61126  11.25                                                        5       121.13110 3.00     1.64129                                                                              55.5                                          6       32.31767  55.49                                                        7       148.79356 10.00    1.81264                                                                              25.5                                          8       -96.99648 1.00                                                         9       INF       35.81                 AST                                    10      -73.31168 3.03     1.81264                                                                              25.5                                          11      39.37385  18.00    1.73234                                                                              54.7                                          12      -61.28330 0.20                                                         13      297.62443 6.07     1.69980                                                                              55.5                                          14      -170.03076                                                                               0.20                                                         15      87.18402  7.64     1.69980                                                                              55.5                                          16      INF       5.00                                                         17      INF       51.00    1.51872                                                                              64.2                                          18      INF                                                                    ______________________________________                                    

First Invention, Embodiment 7

FIG. 15 shows an embodiment 7. Numerical data of this embodiment are shown in TABLE-7. In this embodiment, the spacing d6 (=D1) between the first lens group G1 and the second lens group G2 is set at 50.44 mm, thereby providing the air interval smaller than the one in the embodiment 5 by 5.01 mm. FIG. 16 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 15 and the light beam is caused to pass the lens system. In FIG. 16, the distance between the center (C1) of the last surface of the first lens group G1 and the center (C2) of the mirror M is set at 32.0 mm. The diaphragm AST is arranged in close contact with the small conjugate side surface of lens element L4 constituting the second lens group G2 (the ninth surface in TABLE-7). It can be seen from FIG. 16 that the mirror M can be inserted easily between the first lens group G1 and the second lens group G2.

                  TABLE 7                                                          ______________________________________                                         (EMBODIMENT 7)                                                                 f = 24.00  θ = 43.28°  F = 4.5  β = 23.5                     f1 = -38.93  f2 = 71.91  f3 = 61.38  D1 = 50.44                                D1/f = 2.10  f2/f3 = 1.17  |f1| /f = 1.62                    m       ri        di       ni    νi                                         ______________________________________                                         1       136.53513 12.00    1.69980                                                                              55.5                                          2       980.12571 0.09                                                         3       129.33447 3.00     1.61686                                                                              37.0                                          4       31.19552  9.79                                                         5       94.73813  3.00     1.64129                                                                              55.5                                          6       29.10866  50.44                                                        7       671.82960 16.86    1.81264                                                                              25.5                                          8       -63.28762 0.00                                                         9       INF       41.47                 AST                                    10      -100.80370                                                                               3.03     1.81264                                                                              25.5                                          11      40.30690  18.00    1.73234                                                                              54.7                                          12      -63.45385 0.10                                                         13      705.07045 6.07     1.69980                                                                              55.5                                          14      -133.81413                                                                               0.10                                                         15      117.24205 7.64     1.69980                                                                              55.5                                          16      INF       5.00                                                         17      INF       51.00    1.51872                                                                              64.2                                          18      INF                                                                    ______________________________________                                    

First Invention, Embodiment 8

FIG. 17 shows an embodiment 8. Numerical data of this embodiment are shown in TABLE-8. In this embodiment, the spacing d6 (=D1) between the first lens group G1 and the second lens group G2 is set at 45.13 mm, thereby providing the air interval smaller than the one in the embodiment 5 by 10.32 mm. FIG. 18 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 17 and the light beam is caused to pass the lens system. In FIG. 18, the distance between the center (C1) of the last surface of the first lens group G1 and the center (C2) of the mirror M is set at 28.0 mm. The diaphragm AST is arranged in close contact with the small conjugate side surface of lens element L4 constituting the second lens group G2 (the ninth surface in TABLE-8). It can be seen From FIG. 18 that the mirror M can be mounted between the first lens group G1 and the second lens group G2 in a compact manner.

                  TABLE 8                                                          ______________________________________                                         (EMBODIMENT 8)                                                                 f = 24.00  θ = 43.29°  F = 4.5  β = 23.5                     f1 = -38.98  f2 = 72.76  f3 = 61.57  D1 = 45.13                                D1/f = 1.88  f2/f3 = 1.18  |f1| /f = 1.62                    m       ri        di       ni    νi                                         ______________________________________                                         1       135.58642 12.00    1.69980                                                                              55.5                                          2       874.87953 0.09                                                         3       126.33461 2.84     1.61686                                                                              37.0                                          4       30.51499  10.38                                                        5       98.05461  2.84     1.64129                                                                              55.5                                          6       30.15873  45.13                                                        7       723.13709 27.18    1.81264                                                                              25.5                                          8       -63.30892 0.00                                                         9       INF       41.97                 AST                                    10      -109.36421                                                                               3.03     1.81264                                                                              25.5                                          11      40.89621  18.00    1.73234                                                                              54.7                                          12      -66.03057 0.10                                                         13      569.43909 6.07     1.69980                                                                              55.5                                          14      -131.03458                                                                               0.10                                                         15      124.74636 7.64     1.69980                                                                              55.5                                          16      INF       5.00                                                         17      INF       51.00    1.51872                                                                              64.2                                          18      INF                                                                    ______________________________________                                    

First Invention, Embodiment 9

FIG. 19 shows an embodiment 9. Numerical data of this embodiment are shown in TABLE-9. FIG. 20 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 19 and the light beam is caused to pass the lens system. In FIG. 20, the distance between a center (C1) of the last surface of the first lens group G1 and a center (C2) of the mirror M is set at 60.0 mm. The diaphragm AST is arranged behind the small conjugate side surface of lens element L4 constituting the second lens group G2 away therefrom by 3.3 mm (the ninth surface in TABLE-9). It can be seen from FIG. 20 that a sufficient air interval is attained to insert the mirror M between the first lens group G1 and the second lens group G2. The oblique angle of the chief ray was 1° or smaller, thus a satisfactory telecentric characteristic can be obtained. It can be also seen from FIG. 20 that the oblique angle of the off-axis beam with respect to the optical axis becomes smaller than the outgoing beam does on the screen side (on the large conjugate side) in an interior of the lens system. Accordingly, the mirror M is allowed to have a smaller size than a case where it is arranged outside the projection lens (on the screen side). There is obtained the peripheral illuminance ratio of 55% at a maximum projection half-angle because of effects of the off-axis pupil aberration. Generally, the peripheral luminance ratio of 30% is required in the projection-type display apparatus. The above value sufficiently satisfactory compared to this value. Further, the aspherical lens element L8 has a weak power at the center thereof and a strong negative power at a peripheral portion thereof, thereby being capable of correcting the negative distortion at the peripheral portion when viewed on the side of the liquid crystal panel 3 and the higher-order astigmatism in a well-balanced manner.

In a construction including embodiments 10 to 20, and 25 wherein the aspherical lens is included as the last lens element L8 of the third lens group G3, it is preferable that a ratio of the focal length F4 of the lens element L8 to the Focal length f of the total lens system satisfies a relationship defined by the following equation (6):

|f4|/f>10 (6).

If the ratio is in excess of the lower limit value of the equation (6), an on-axis refracting power or the aspherical lens element. L8 becomes too strong. Thus, if the lens element L8 is Formed or plastic material, a problem of defocusing of the projected image cannot be ignored which is caused by a change or the environment such as the temperature and the humidity.

                                      TABLE 9                                      __________________________________________________________________________     (EMBODIMENT 9)                                                                 __________________________________________________________________________     f = 41.02  θ = 43.35°  F = 4.5  β = 14.5                     f1 = -83.85  f2 = 118.52  f3 = 88.30  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.34  |f1| /f = 2.04                    f4 = -1718.07  |f4| /f = 41.88                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  209.28882                                                                               18.00 1.69680                                                                               55.5                                                  2  740.90423                                                                               0.20                                                               3  190.31778                                                                               5.00  1.61293                                                                               37.0                                                  4  59.32194 21.02                                                              5  338.42940                                                                               4.00  1.63854                                                                               55.4                                                  6  69.63940 95.00                                                              7  320.89460                                                                               14.00 1.69894                                                                               30.1                                                  8  -110.81612                                                                              3.30                                                               9  INF      55.46              AST                                             10 -60.17964                                                                               10.83 1.84666                                                                               23.8                                                  11 162.62940                                                                               26.00 1.69680                                                                               55.5                                                  12 -59.95669                                                                               0.20                                                               13 126.56120                                                                               23.00 1.62299                                                                               58.1                                                  14 -141.09431                                                                              0.20                                                               15 -515.53888                                                                              3.00  1.49154                                                                               57.8  *                                               16 -1317.55068                                                                             20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4    A6     A8     A10                                              __________________________________________________________________________     15 101.211102                                                                            0.253221E-7                                                                          0.666013E-10                                                                          0.925652E-14                                                                          0.720830E-17                                     16 -2000.000000                                                                          0.389066E-6                                                                          0.457664E-10                                                                          0.361257E-13                                                                          0.444744E-17                                     __________________________________________________________________________

First Invention, Embodiments 10 to 12

Numerical data of embodiments 10 to 12 are shown in TABLES-10 to 12. Sectional views showing lens systems of the respective embodiments are substantially similar to FIG. 19 showing the embodiment 9. In these embodiments, the air interval about the same as the one in the embodiment 9 is attained between the first lens group G1 and the second lens group G2, and accordingly there is no problem in inserting the mirror M.

                                      TABLE 10                                     __________________________________________________________________________     (EMBODIMENT 10)                                                                __________________________________________________________________________     f = 41.01  θ = 43.25°  F = 4.5  β = 14.5                     f1 = -78.55  f2 = 116.78  f3 = 92.35  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.26  |f1| /f = 1.92                    f4 = -1846.54  |f4| /f = 45.03                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  214.09482                                                                               18.00 1.69680                                                                               55.5                                                  2  910.92980                                                                               0.10                                                               3  193.62553                                                                               5.00  1.61293                                                                               37.0                                                  4  56.62424 21.52                                                              5  358.74711                                                                               4.00  1.63854                                                                               55.4                                                  6  68.54356 95.00                                                              7  275.62302                                                                               14.00 1.69894                                                                               30.1                                                  8  -114.80012                                                                              3.31                                                               9  INF      60.70              AST                                             10 -56.84164                                                                               4.34  1.84666                                                                               23.8                                                  11 191.34903                                                                               26.00 1.69680                                                                               55.5                                                  12 -57.44741                                                                               0.10                                                               13 122.68404                                                                               23.00 1.62299                                                                               58.1                                                  14 -141.10233                                                                              0.10                                                               15 -509.36767                                                                              3.00  1.49154                                                                               57.8  *                                               16 -1156.99893                                                                             20.00              *                                               17  INF     71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4    A6     A8     A10                                              __________________________________________________________________________     15 102.52516                                                                             0.343121E-7                                                                          0.765024E-10                                                                          0.105031E-13                                                                          0.924494E-17                                     16 -2000.000000                                                                          0.405307E-6                                                                          0.541539E-10                                                                          0.452024E-13                                                                          0.534341E-17                                     __________________________________________________________________________

                                      TABLE 11                                     __________________________________________________________________________     (EMBODIMENT 11)                                                                __________________________________________________________________________     f = 41.02  θ = 43.31°  F = 4.5  β = 14.5                     f1 = -80.97  f2 = 117.92  f3 = 90.62  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.30  |f1| /f = 1.97                    f4 = -1836.13  |f4| /f = 44.76                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  207.13526                                                                               18.00 1.69680                                                                               55.5                                                  2  758.25591                                                                               0.10                                                               3  185.17029                                                                               5.00  1.61293                                                                               37.0                                                  4  58.14060 22.16                                                              5  469.20601                                                                               4.00  1.63854                                                                               55.4                                                  6  71.34215 95.00                                                              7  278.16884                                                                               14.00 1.69894                                                                               30.1                                                  8  -115.96523                                                                              3.31                                                               9  INF      58.35              AST                                             10 -57.07889                                                                               5.68  1.84666                                                                               23.8                                                  11 186.26313                                                                               26.16 1.69680                                                                               55.5                                                  12 -57.43884                                                                               0.10                                                               13 123.85803                                                                               23.00 1.62299                                                                               58.1                                                  14 -141.04934                                                                              0.10                                                               15 -508.72473                                                                              3.00  1.49154                                                                               57.8  *                                               16 -1162.01899                                                                             20.00              *                                               17  INF     71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4    A6     A8     A10                                              __________________________________________________________________________     15 102.60548                                                                             0.326666E-7                                                                          0.760660E-10                                                                          0.105266E-13                                                                          0.918507E-17                                     16 -2000.000000                                                                          0.408050E-6                                                                          0.535449E-10                                                                          0.445331E-13                                                                          0.517802E-17                                     __________________________________________________________________________

                                      TABLE 12                                     __________________________________________________________________________     (EMBODIMENT 12)                                                                __________________________________________________________________________     f = 41.01  θ = 43.25°  F = 4.5  β = 14.5                     f1 = -77.81  f2 = 116.41  f3 = 92.57  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.26  |f1| /f = 1.90                    f4 = -1987.38 |f4| /f = 48.46                                m  ri       di    ni     νi                                                 __________________________________________________________________________     1  218.48840                                                                               17.00 1. 69680                                                                              55.5                                                  2  1028.03053                                                                              0.10                                                               3  196.04580                                                                               5.00  1.61293                                                                               37.0                                                  4  57.99843 21.50                                                              5  425.30204                                                                               4.00  1.63854                                                                               55.4                                                  6  68.23706 95.00                                                              7  240.48781                                                                               14.00 1.69894                                                                               30.1                                                  8  -121.43348                                                                              3.31                                                               9  INF      60.91              AST                                             10 -56.74421                                                                               3.91  1.84666                                                                               23.8                                                  11 192.38945                                                                               26.00 1.69680                                                                               55.5                                                  12 -57.85630                                                                               0.10                                                               13 120.39684                                                                               23.00 1.62299                                                                               58.1                                                  14 -141.16501                                                                              0.10                                                               15 -510.31629                                                                              3.00  1.49154                                                                               57.8  *                                               16 -1065.78653                                                                             20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4    A6     A8     A10                                              __________________________________________________________________________     15 102.380758                                                                            0.349514E-7                                                                          0.760766E-10                                                                          0.102687E-13                                                                          0.926386E-17                                     16 -2000.000000                                                                          0.403383E-6                                                                          0.559040E-10                                                                          0.461159E-13                                                                          0.554591E-17                                     __________________________________________________________________________

First Invention, Embodiment 13

FIG. 21 is a sectional view showing a lens system of an embodiment 13. Numerical data of this embodiment are shown in TABLE-13. In this embodiment as well, the air interval about the same as the one in the embodiment 9 is attained between the first lens group G1 and the second lens group G2, thereby allowing the mirror M to be inserted. Further, in this embodiment, the aberration is corrected in a state where a PMMA resin plate L9 having the thickness of 70 mm and a glass plate L10 (BSC7: manufactured by HOYA Kabushiki Kaisha) having the thickness of 1 mm are inserted as a parallel plane plate arranged after the third lens group G3. This supposes that the dichroic prism for color composition is formed of the PMMA resin to be described later in relation to the fourth invention. Further, the glass plate having the thickness of 1 mm supposes a glass substrate for enclosing the liquid crystal in the liquid crystal panel.

The retro-focus type lens according to the invention is capable of correcting the aberrations satisfactorily even if the dichroic prism is formed of plastic material as in this embodiment, and contributes to realization of low cost and light weighted projection-type display apparatuses. Since the parallel plane plate L9 equivalent to the dichroic prism has no refracting power, a change in the refractive index of the plastic material due to the change in the environment such as the temperature and the humidity does not give a large influence on the defocusing of the projected image. Besides PMMA, as plastic material can be used, for example, PC, ZEONEX (manufactured by Nippon Zeon Kabushiki Kaisha), OZ1000 (manufactured by Hitachi Kasei Kogyo Kabushiki Kaisha), ARTON (manufactured by Nippon Gohsei Gomu Kabushiki Kaisha), and APO (Mitsui Sekiyu Kagaku Kogyo Kabushiki Kaisha). Although the thickness of the plastic parallel plane plate L9 is set at 70 mm in this embodiment, it is adjustable according to the screen size of the liquid cryshal panel 3 as will be clear from FIGS. 1 and 2 showing the conventional projection-type display apparatuses. Accordingly, even if the thickness of the parallel plane plate L9 is set at 70 mm or smaller (e.g., about 15 to 70 mm), the aberration correction can be carried out sufficiently by the construction similar to the lens system of this embodiment. Further, even if the thickness of the parallel plane plate L9 is in excess of 70 mm, the aberrations can be corrected in a similar manner so long as there is no problem in ranging the liquid crystal panel 3. In other words, even if the parallel plane plate is formed of plastic material in 90% of its entire thickness, the retro-focus type lens according to the invention is capable of correcting the aberrations sufficiently.

                                      TABLE 13                                     __________________________________________________________________________     (EMBODIMENT 13)                                                                __________________________________________________________________________     f = 41.00  θ = 43.34°  F = 4.5  β = 14.5                     f1 = -84.12  f2 = 119.57  f3 = 88.29  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.35  |f1| /f = 2.05                    f4 = -1703.02  |f4| /f = 41.54                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  208.10985                                                                               18.00 1.69680                                                                               55.5                                                  2  760.71289                                                                               0.20                                                               3  193.50540                                                                               5.00  1.61293                                                                               37.0                                                  4  59.27085 21.03                                                              5  342.67312                                                                               4.00  1.63854                                                                               55.4                                                  6  70.22761 95.00                                                              7  334.13001                                                                               14.00 1.69894                                                                               30.1                                                  8  -110.66841                                                                              3.30                                                               9  INF      54.88              AST                                             10 -61.04168                                                                               11.64 1.84666                                                                               23.8                                                  11 158.38334                                                                               26.00 1.69680                                                                               55.5                                                  12 -60.48160                                                                               0.20                                                               13 128.04196                                                                               23.00 1.62299                                                                               58.1                                                  14 -141.07358                                                                              0.20                                                               15 -517.11596                                                                              3.00  1.49154                                                                               57.8  *                                               16 -1346.4762                                                                              20.00              *                                               17  INF     70.00 1.49154                                                                               57.8                                                  18 INF      1.00  1.51680                                                                               64.2                                                  19 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4    A6     A8     A10                                              __________________________________________________________________________     15 100.776415                                                                            0.283045E-7                                                                          0.646495E-10                                                                          0.912131E-14                                                                          0.686409E-17                                     16 -2000.000000                                                                          0.380230E-6                                                                          0.447362E-10                                                                          0.344526E-13                                                                          0.432562E-17                                     __________________________________________________________________________

First Invention, Embodiments 14 to 20

Numerical data of embodiments 14 to 20 are shown in TABLES-14 to 20. The lens systems of these embodiments are unillustrated since sectional views showing the same are analogous to FIG. 19 showing the embodiment 9. In these embodiments as well, the air interval about the same as the one in the embodiment 9 is attained between the first lens group G1 and the second lens group G2, thereby creating no problem in inserting the mirror M.

                                      TABLE 14                                     __________________________________________________________________________     (EMBODIMENT 14)                                                                __________________________________________________________________________     f = 41.02  θ = 43.41°  F = 4.5  β = 14.5                     f1 =  80.79  f2 = 131.75  f3 =  90.42  D1 =  93.00                             D1/f =  2.27  f2/f3 = 1.46  |f1| /f = 1.97                   f4 = -813.27  |f4| /f = 19.83                                m  ri       di    ni     νi                                                 __________________________________________________________________________     1  247.63993                                                                               17.00 1.69680                                                                               55.5                                                  2  869.53816                                                                               0.10                                                               3  180.49687                                                                               5.00  1.61293                                                                               37.0                                                  4  59.15650 21.82                                                              5  394.02309                                                                               4.00  1.63854                                                                               55.4                                                  6  1.68554  93.00                                                              7  779.91992                                                                               14.00 1.80518                                                                               25.5                                                  8  -123.10690                                                                              11.71                                                              9  INF      62.12              AST                                             10 -61.94791                                                                               2.80  1.80518                                                                               25.5                                                  11 -132.49952                                                                              25.50 1.71300                                                                               53.9                                                  12 -60.68283                                                                               0.10                                                               13 128.02330                                                                               24.23 1.67790                                                                               55.5                                                  14 -144.33425                                                                              0.12                                                               15 -545.16245                                                                              3.58  1.49154                                                                               57.8  *                                               16 1525.448159                                                                             20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 102.000000                                                                            -0.135920E-6                                                                          0.544483E-10                                                                          0.511768E-14                                                                          0.424992E-17                                    16 -2000.000000                                                                           0.395527E-6                                                                          0.129353E-10                                                                          0.257261E-13                                                                          0.592184E-17                                    __________________________________________________________________________

                                      TABLE 15                                     __________________________________________________________________________     (EMBODIMENT 15)                                                                __________________________________________________________________________     f = 41.00  θ = 43.46°  F = 4.5  β = 14.5                     f1 = -85.37  f2 = 139.19  f3 = 88.93  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.57  |f1| /f = 2.08                    f4 = -1997.50  |f4| /f = 48.72                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  223.60991                                                                               18.00 1.69680                                                                               55.5                                                  2  815.85357                                                                               0.20                                                               3  218.87795                                                                               5.00  1.61293                                                                               37.0                                                  4  60.82318 21.82                                                              5  427.20443                                                                               4.00  1.63854                                                                               55.4                                                  6  8.31506  95.00                                                              7  1842.49361                                                                              14.00 1.80518                                                                               25.5                                                  8  -120.09981                                                                              12.00                                                              9  INF      62.60              AST                                             10 -74.15737                                                                               3.00  1.80518                                                                               25.5                                                  11 112.57176                                                                               26.00 1.71300                                                                               53.9                                                  12 -63.64452                                                                               0.20                                                               13 173.92879                                                                               23.00 1.67790                                                                               55.5                                                  14 -137.91210                                                                              0.20                                                               15 -519.52669                                                                              3.00  1.49154                                                                               57.8  *                                               16 -1100.36498                                                                             20.00              *                                               17  INF     71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 90.479412                                                                             -0.943206E-8                                                                          0.289802E-10                                                                          0.547844E-14                                                                          0.566579E-17                                    16 -2000.000000                                                                           0.246380E-6                                                                          0.380317E-10                                                                          0.206938E-13                                                                          0.367889E-17                                    __________________________________________________________________________

                                      TABLE 16                                     __________________________________________________________________________     (EMBODIMENT 16)                                                                __________________________________________________________________________     f = 41.00  θ = 43.35°  F = 4.5  β = 14.5                     f1 = -84.86  f2 = 135.66  f3 = 87.65  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.55  |f1| /f = 2.07                    f4 = -1662.67  |f4| /f = 40.55                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  257.41023                                                                               12.84 1.69680                                                                               55.5                                                  2  1130.80334                                                                              0.10                                                               3  179.27669                                                                               2.80  1.61293                                                                               37.0                                                  4  61.14283 20.93                                                              5  318.65098                                                                               2.80  1.63854                                                                               55.4                                                  6  70.92954 95.00                                                              7  615.02360                                                                               8.00  1.80518                                                                               25.5                                                  8  -133.53002                                                                              14.15                                                              9  INF      61.07              AST                                             10 -60.41649                                                                               2.80  1.80518                                                                               25.5                                                  11 149.20663                                                                               22.88 1.71300                                                                               53.9                                                  12 -58.65602                                                                               0.10                                                               13 128.37404                                                                               19.03 1.67790                                                                               55.5                                                  14 -152.80432                                                                              0.10                                                               15 -722.06424                                                                              3.00  1.49154                                                                               57.8  *                                               16 -6019.57696                                                                             20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 199.700459                                                                            -0.963286E-7                                                                          0.470225E-10                                                                          0.668325E-14                                                                          0.736259E-17                                    16 -1000.000000                                                                           0.333081E-6                                                                          0.343574E-10                                                                          0.293414E-13                                                                          0.449826E-17                                    __________________________________________________________________________

                                      TABLE 17                                     __________________________________________________________________________     (EMBODIMENT 17)                                                                __________________________________________________________________________     f = 41.00  θ = 43.30°  F = 4.5  β = 14.5                     f1 = -81.76  f2 = 129.91  f3 = 89.73  D1 = 93.00                               D1/f = 2.27  f2/f3 = 1.45  |f1| /f = 1.99                    f4 = -829.17  |f4|/f = 20.22                                 m  ri       di    ni     νi                                                 __________________________________________________________________________     1  241. 86167                                                                              17.00 1.69680                                                                               55.5                                                  2  1042.70429                                                                              0.20                                                               3  197.11785                                                                               5.00  1.61293                                                                               37.0                                                  4  60.44744 21.03                                                              5  352.58126                                                                               4.00  1.63854                                                                               55.4                                                  6  69.85648 93.00                                                              7  637.35510                                                                               14.00 1.80518                                                                               25.5                                                  8  -125.28619                                                                              11.45                                                              9  INF      59.18              AST                                             10 -60.25897                                                                               3.00  1.80518                                                                               25.5                                                  11 132.82538                                                                               25.50 1.71300                                                                               53.9                                                  12 -58.94358                                                                               0.20                                                               13 120.98851                                                                               22.00 1.65020                                                                               55.7                                                  14 -145.87655                                                                              0.20                                                               15 -713.93564                                                                              3.00  1.52540                                                                               56.3  *                                               16 1131.46191                                                                              20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 200.952916                                                                            -0.106096E-6                                                                          0.589142E-10                                                                          0.736031E-14                                                                          0.701461E-17                                    16 404.800069                                                                             0.374221E-6                                                                          0.190386E-10                                                                          0.310185E-13                                                                          0.670009E-17                                    __________________________________________________________________________

                                      TABLE 18                                     __________________________________________________________________________     (EMBODIMENT 18)                                                                __________________________________________________________________________     f = 41.00  θ = 43.33°  F = 4.5  β = 14.5                     f1 = -82.33  f2 = 131.31  f3 = 89.29  D1 = 93.00                               D1/f = 2.27  f2/f3 = 1.47  |f1| /f = 2.01                    f4 = -878.85  |f4|/f = 21.44                                 m  ri       di    ni     νi                                                 __________________________________________________________________________     1  246.28621                                                                               17.00 1.69680                                                                               55.5                                                  2  1088.41351                                                                              0.20                                                               3  206.70541                                                                               5.00  1.61293                                                                               37.0                                                  4  61.27672 20.63                                                              5  324.37105                                                                               4.00  1.63854                                                                               55.4                                                  6  69.56946 93.00                                                              7  718.53096                                                                               14.00 1.80518                                                                               25.5                                                  8  -124.22263                                                                              12.71                                                              9  INF      58.98              AST                                             10 -60.81688                                                                               3.00  1.80518                                                                               25.5                                                  11 134.68545                                                                               25.50 1.71300                                                                               53.9                                                  12 -58.81675                                                                               0.20                                                               13 128.92691                                                                               22.00 1.67790                                                                               55.5                                                  14 -152.22962                                                                              0.20                                                               15 -717.64984                                                                              3.21  1.52540                                                                               56.3  *                                               16 1312.42964                                                                              20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 200.354009                                                                            -0.110832E-6                                                                          0.536294E-10                                                                          0.656798E-14                                                                          0.677723E-17                                    16 321.788587                                                                             0.354296E-6                                                                          0.212024E-10                                                                          0.291269E-13                                                                          0.566620E-17                                    __________________________________________________________________________

                                      TABLE 19                                     __________________________________________________________________________     (EMBODIMENT 19)                                                                __________________________________________________________________________     f = 41.00  θ = 43.33°  F = 4.5  β = 14.5                     f1 = -84.63  f2 = 134.96  f3 = 88.10  D1 = 95.00                               D1/f = 2.32  f2/f3 = 1.53  |f1| /f = 2.06                    f4 = -1507.90  |f4| /f = 36.78                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  263.71173                                                                               13.18 1.69680                                                                               55.5                                                  2  1494.08116                                                                              0.10                                                               3  197.25859                                                                               2.80  1.61293                                                                               37.0                                                  4  62.39665 19.93                                                              5  300.34894                                                                               2.80  1.63854                                                                               55.4                                                  6  69.80455 95.00                                                              7  538.93483                                                                               10.26 1.80518                                                                               25.5                                                  8  -136.51341                                                                              13.07                                                              9  INF      61.57              AST                                             10 -60.56141                                                                               2.80  1.80518                                                                               25.5                                                  11 144.95228                                                                               22.89 1.71300                                                                               53.9                                                  12 -58.72942                                                                               0.10                                                               13 128.05226                                                                               18.96 1.67790                                                                               55.5                                                  14 -152.72486                                                                              0.10                                                               15 -729.88175                                                                              3.00  1.52540                                                                               56.3  *                                               16 -8848.89019                                                                             20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 198.348669                                                                            -0.103625E-6                                                                          0.479375E-10                                                                          0.565819E-14                                                                          0.660449E-17                                    16 -1000.000000                                                                           0.317395E-6                                                                          0.242825E-10                                                                          0.277963E-13                                                                          0.471735E-17                                    __________________________________________________________________________

                                      TABLE 20                                     __________________________________________________________________________     (EMBODIMENT 20)                                                                __________________________________________________________________________     f = 41.00  θ = 43.27°  F = 4.5  β= 14.5                      f1 = -81.96  f2 = 127.70  f3 = 90.83  D1 = 97.00                               D1/f = 2.37  f2/f3 = 1.41  |f1|/f = 2.00                     f4 = -3297.50  |f4| /f = 80.43                               m  ri       di    ni     νi                                                 __________________________________________________________________________     1  217.01481                                                                               13.73 1.69680                                                                               55.5                                                  2  823.26087                                                                               0.10                                                               3  155.37376                                                                               2.20  1.61293                                                                               37.0                                                  4  57.95555 21.52                                                              5  318.80893                                                                               2.20  1.63854                                                                               55.4                                                  6  65.59215 97.00                                                              7  635.45623                                                                               7.00  1.80518                                                                               25.5                                                  8  -123.42112                                                                              5.29                                                               9  INF      65.49              AST                                             10 -66.86998                                                                               2.20  1.80518                                                                               25.5                                                  11 113.84212                                                                               24.08 1.71300                                                                               53.9                                                  12 -60.77909                                                                               0.10                                                               13 142.83715                                                                               16.21 1.69680                                                                               55.5                                                  14 -180.08480                                                                              0.10                                                               15 -1125.93847                                                                             2.20  1.52540                                                                               56.3  *                                               16 -3193.16731                                                                             20.00              *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 205.302614                                                                            -0.143209E-6                                                                          0.436306E-10                                                                          0.352016E-14                                                                          0.459394E-17                                    16 2243.146704                                                                            0.301114E-6                                                                          0.395417E-11                                                                          0.212312E-13                                                                          0.505731E-17                                    __________________________________________________________________________

First Invention, Embodiment 21

FIG. 22 is a sectional view of a lens system of an embodiment 21. Numerical data of the embodiment 21 are shown in TABLE-21. A sufficiently large air interval is attained between the first lens group G1 and the second lens group G2, thereby creating no problem in inserting the mirror M. In this embodiment, the first lens group G1 includes four lens elements, and the lens element L0 on the large conjugate side is a bi-aspherical lens having a weak refracting power on the optical axis and a strong positive refracting power at a peripheral portion thereof. The lens element L0 mainly corrects the off-axis distortion. The aspherical lens element L0 is preferably such that a ratio of the focal length f5 of the lens element L0 to the focal length F of the total lens system satisfies a relationship defined by the following equation (7) in this embodiment as well as an embodiment 22 to be described later:

|f5|/f>100 (7).

If the ratio is in excess of the lower limit value of the equation (7), the on-axis refracting power of the aspherical lens element L0 becomes too strong. Thus, when the lens element L0 is formed of plastic material, a problem of defocusing of the projected image cannot be ignored which is caused by a change of the environment such as the temperature and the humidity.

                                      TABLE 21                                     __________________________________________________________________________     (EMBODIMENT 21)                                                                __________________________________________________________________________     f = 41.00  θ = 42.55°  F = 4.5  β= 14.5                      f1 = -72.66  f2 = 135.07  f3 = 107.46  D1 = 104.43                             D1/f = 2.55  f2/f3 = 1.26  |f1| /f = 1.77                    f5 = 5.82E3  f5/f = 142                                                        m  ri       di    ni     νi                                                 __________________________________________________________________________     1  817.86370                                                                               23.95 1.49154                                                                               57.8  *                                               2  1132.05530                                                                              0.99               *                                               3  342.82557                                                                               17.00 1.69680                                                                               55.5                                                  4  3105.23658                                                                              0.22                                                               5  4134.01653                                                                              6.00  1.61293                                                                               37.0                                                  6  58.61559 16.88                                                              7  139.01296                                                                               5.00  1.63854                                                                               55.4                                                  8  65.35020 104.43                                                             9  339.98114                                                                               30.00 1.80518                                                                               25.5                                                  10 -155.67589                                                                              8.11                                                               11 INF      64.91              AST                                             12 -135.31259                                                                              19.58 1.80518                                                                               25.5                                                  13 77.61582 23.00 1.72916                                                                               54.7                                                  14 -103.27451                                                                              0.20                                                               15 1141.72544                                                                              17.28 1.69680                                                                               55.5                                                  16 -229.34545                                                                              0.20                                                               17 197.29088                                                                               24.94 1.69680                                                                               55.5                                                  18 17773.90764                                                                             20.00                                                              19 INF      71.00 1.51680                                                                               64.2                                                  20 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     1  43.136111                                                                             0.108317E-6                                                                           -0.282015E-11                                                                         -0.240621E-16                                                                         0.374139E-20                                    2  -6.226369                                                                             0.660407E-9                                                                           -0.118567E-12                                                                         -0.386260E-17                                                                         0.155721E-20                                    __________________________________________________________________________

First Invention, Embodiment 22

Numerical data of the embodiment 22 are shown in TABLE-22. A sectional view of a lens system of this embodiment is analogous to FIG. 22 showing the embodiment 21. In this embodiment as well, a sufficiently large air interval is attained between the first lens group G1 and the second lens group G2, thereby creating no problem in inserting the mirror M. Further in this embodiment, only the large conjugate side surface of the lens element L0 of the first lens group G1 is formed into an aspherical surface, thereby mainly correcting the off-axis distortion.

                                      TABLE 22                                     __________________________________________________________________________     (EMBODIMENT 22)                                                                __________________________________________________________________________     f = 41.00  θ = 42.79°  F = 4.5  β = 14.5                     f1 = -72.15  f2 = 133.23  f3 = 107.69  D1 = 104.66                             D1/f = 2.55  f2/f3 = 1.24  |f1| /f = 1.76                    f5 = 2.50E5  f5/f = 6.10E3                                                     m  ri       di    ni     νi                                                 __________________________________________________________________________     1  809.71244                                                                               16.07 1.49154                                                                               57.8  *                                               2  809.71734                                                                               0.99                                                               3  220.68091                                                                               17.06 1.69680                                                                               55.5                                                  4  965.56137                                                                               0.20                                                               5  1033.03613                                                                              5.08  1.61293                                                                               37.0                                                  6  59.85684 18.67                                                              7  162.76270                                                                               4.49  1.63854                                                                               55.4                                                  8  61.41639 104.66                                                             9  256.85666                                                                               30.00 1.80518                                                                               25.5                                                  10 -177.35439                                                                              1.63                                                               11 INF      67.56              AST                                             12 -123.58094                                                                              12.08 1.80518                                                                               25.5                                                  13 73.62199 22.25 1.72916                                                                               54.7                                                  14 -99.09597                                                                               0.20                                                               15 858.68817                                                                               22.98 1.69680                                                                               55.5                                                  16 -212.31665                                                                              0.20                                                               17 199.27482                                                                               21.65 1.69680                                                                               55.5                                                  18 INF      20.00                                                              19 INF      71.00 1.51680                                                                               64.2                                                  20 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K    A4    A6      A8      A10                                              __________________________________________________________________________     1  46.036429                                                                           0.103407E-6                                                                          -0.230100E-11                                                                          -0.388917E-16                                                                          0.130263E-20                                     __________________________________________________________________________

First Invention, Embodiment 23

FIG. 23 is a sectional view of a lens system of an embodiment 23. Numerical data of this embodiment are shown in TABLE-23. In this embodiment, an air interval far greater than those in the embodiments 21, 22 is attained between the first lens group G1 and the second lens group G2, thus there is no problem in inserting the mirror M. Further, the lens element L3 of the first lens group G1 on the side of the liquid crystal panel 3 and the lens element L4 of the second lens group G2 are an aspherical lens each, thereby correcting the on-axis spherical aberration, off-axis distortion, and high-order astigmatism in a highly well-balanced manner. Since the lens elements L3, L4 having relatively small diameters are formed into the aspherical lens in this embodiment, it is easy to form the aspherical surface.

                                      TABLE 23                                     __________________________________________________________________________     (EMBODIMENT 23)                                                                __________________________________________________________________________     f = 40.00  θ = 42.16°  f = 4.5  β = 14.5                     f1 = -69.13  f2 = 130.85  f3 = 113.66  D1 = 107.92                             D1/f = 2.70  f2/f3 = 1.15  |f1| /f = 1.73                    m  ri      di      ni      νi                                               __________________________________________________________________________     1  180.68905                                                                              22.00   1.69680 55.5                                                2  206.04152                                                                              0.20                                                                3  110.37469                                                                              5.00    1.61293 37.0                                                4  53.37385                                                                               16.72                                                               5  104.95392                                                                              4.03    1.52540 56.3    *                                           6  36.18085                                                                               107.92                  *                                           7  438.02979                                                                              35.00   1.49154 57.8    *                                           8  -73.74918                                                                              1.00                    *                                           9  INF     88.88                   AST                                         10 -259.71093                                                                             5.28    1.80518 25.5                                                11 77.08544                                                                               21.50   1.72916 54.7                                                12 -130.52172                                                                             0.20                                                                13 966.10879                                                                              7.50    1.69680 55.5                                                14 -290.87101                                                                             0.20                                                                15 192.38328                                                                              8.00    1.69680 55.5                                                16 8665.28884                                                                             20.00                                                               17 INF     71.00   1.51680 64.2                                                18 INF                                                                         __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6      A8      A10                                           __________________________________________________________________________     5  -0.768377                                                                              0.336042E-6                                                                          -0.167690E-9                                                                           -0.535577E-13                                                                          0.318776E-16                                  6  -0.387362                                                                             -0.771756E-6                                                                          0.311739E-9                                                                            -0.108533E-11                                                                          0.239659E-15                                  7  -221.366534                                                                           -0.812804E-6                                                                          0.572113E-8                                                                            -0.203842E-10                                                                          0.249052E-13                                  8  0.73206                                                                                0.177862E-6                                                                          0.119981E-8                                                                             0.168784E-11                                                                          -0.251716E-13                                 __________________________________________________________________________

First Invention, Embodiment 24

FIG. 24 shows an embodiment 24. Numerical data of this embodiment are shown in TABLE-24. FIG. 25 shows an example in which the bent mirror M inclined at 45° is inserted in the same construction as in FIG. 24 and the light beam is caused to pass the lens system. In FIG. 25, the distance between the center (C1) of the last surface of the first lens group G1 and the center (C2) of the mirror M is set at 56.23 mm. The diaphragm AST is arranged behind the small conjugate side surface of lens element L4 constituting the second lens group G2 away therefrom by 1.64 mm (the ninth surface in TABLE-24). It can be seen that the mirror M can be inserted in a relatively small air interval between the first lens group G1 and the second lens group G2 in a compact manner. In this embodiment, all the lens elements are spherical lenses, and accordingly the distortion is relatively large as shown in an aberration diagram later. However, this lens system has a sufficient performance for an application, for example, in displaying a TV image, and is manufacturable inexpensively since aspherical lenses are not used.

                  TABLE 24                                                         ______________________________________                                         (EMBODIMENT 24)                                                                f = 41.09  θ = 43.82°  F = 4.5  β = 14.5                     f1 = -72.39  f2 = 127.06  f3 = 102.95  D1 = 88.23                              D1/f = 2.15   f2/f3 = 1.23  |f1| /f = 1.76                   ______________________________________                                         m       ri         di       ni    νi                                        ______________________________________                                         1       230.79216  25.38    1.69680                                                                              55.5                                         2       2311.34846 0.10                                                        3       256.97749  2.40     1.61293                                                                              37.0                                         4       70.07194   16.36                                                       5       375.71609  2.40     1.63854                                                                              55.4                                         6       52.65455   88.23                                                       7       479.91760  50.00    1.80518                                                                              25.5                                         8       -125.41794 1.64                                                        9       INF        64.29                 AST                                   10      -101.85108 13.84    1.80518                                                                              25.5                                         11      85.33577   22.52    1.73400                                                                              51.1                                         12      -91.14216  0.10                                                        13      1107.96055 7.29     1.69350                                                                              50.8                                         14      -250.34875 0.33                                                        15      162.38709  24.48    1.69350                                                                              50.8                                         16      -7552.93986                                                                               20.00                                                       17      INF        71.00    1.51680                                                                              64.2                                         18      INF                                                                    ______________________________________                                    

First Invention, Embodiment 25

FIGS. 26, 27, and 28 shows an embodiment 25. This embodiment shows a zoom lens. The focal length of the zoom lens becomes greater in the order of FIGS. 26, 27, and 28. Numerical data of this embodiment are shown in TABLE-25. FIG. 29(a) to 29(c) show examples in which the bent mirror M inclined at 45° is inserted in the same construction as in FIGS. 26 to 28 and the light beam is caused to pass the respective lens systems. In FIGS. 29, the distance between the center (C1) of the last surface of the first lens group G1 and the center (C2) of the mirror M is set at 53.00 mm. The diaphragm AST is arranged behind the small conjugate side surface of lens element L4 constituting the second lens group G2 away therefrom by 4 to 20 mm (the ninth surface in TABLE-25). It can be seen from FIGS. 29 that the mirror M can be inserted between the first lens group G1 and the second lens group G2.

In this embodiment, in order to change the focal length of the total lens system, the lens element L4 of the second lens group G2 is made movable independently (a, b in TABLE-25). This arrangement makes the focal length of the total lens system variable while retaining the telecentric characteristic on the side of the liquid crystal panel 3. Further in order to compensate for the shift of an image surface on the side of the liquid crystal panel 3 resulting from the change in the focal length, the total lens system including the lens groups G1, G2, and G3 is made movable integrally along the optical axis relative to the parallel plane plate L9 (c in TABLE-25). By the zooming as described above, the projection magnification β can be adjusted from 13.8× to 15.3×, about 11%. A magnification fine adjusting function of about 10% is required in reducing the moire of the projected image to be described later and in adjusting the magnification of a multi-vision projector. The zooming function of the lens system of this embodiment satisfies this requirement. Further, in order to compensate for the shift of the image surface, it may be appropriate to make the lens group G1 movable independently (TABLE-25, fine adjustment of d6 in FIGS. 26 to 28) as well as to make the lens groups G1, G2, and G3 movable integrally. With this arrangement, there can be obtained a further satisfactory imaging characteristic.

                                      TABLE 25                                     __________________________________________________________________________     (EMBODIMENT 25)                                                                __________________________________________________________________________     f = 38.8˜43.2  θ = 44.9˜42.0°  F = 4.5  β =      15.3˜13.8                                                                f1 = -85.37  f2 = 139.19  f3 = 88.93  D1 = 103˜87                        D1/f = 2.65˜2.01  f2/f3 = 1.57  |f1| /f =              2.20˜1.98                                                                f4 = -1997.50 |f4| /f = 50.94˜45.76                    m  ri       di    ni     νi                                                 __________________________________________________________________________     1  223.60991                                                                               18.00 1.69680                                                                               55.5                                                  2  815.85357                                                                               0.20                                                               3  218.87795                                                                               5.00  1.61293                                                                               37.0                                                  4  60.82318 21.82                                                              5  427.20443                                                                               4.00  1.63854                                                                               55.4                                                  6  78.31506 a                                                                  7  1842.49361                                                                              14.00 1.80518                                                                               25.5                                                  8  -120.09981                                                                              b                                                                  9  INF      62.60              AST                                             10 -74.15737                                                                               3.00  1.80518                                                                               25.5                                                  11 112.57176                                                                               26.00 1.71300                                                                               53.9                                                  12 -63.64452                                                                               0.20                                                               13 173.92879                                                                               23.00 1.67790                                                                               55.5                                                  14 -137.91210                                                                              0.20                                                               15 -519.52669                                                                              3.00  1.49154                                                                               57.8  *                                               16 -1100.36498                                                                             c                  *                                               17 INF      71.00 1.51680                                                                               64.2                                                  18 INF                                                                         __________________________________________________________________________     <Variable Distance>                                                                      a            b   c                                                   __________________________________________________________________________     f = 38.8  103.0        4.0 18.26                                               f = 41.0  95.0         12.0                                                                               20.00                                               f = 43.2  87.0         20.0                                                                               22.01                                               __________________________________________________________________________     <Polynominal Deformation Terms>                                                m  K      A4     A6     A8     A10                                             __________________________________________________________________________     15 90.479412                                                                             -0.943206E-8                                                                          0.289802E-10                                                                          0.547844E-14                                                                          0.566579E-17                                    16 -2000.000000                                                                           0.246380E-6                                                                          0.380317E-10                                                                          0.206938E-13                                                                          0.367889E-17                                    __________________________________________________________________________

FIGS. 30(a) to 53(c) are aberration diagrams respectively corresponding to the embodiments 1 to 24 when viewed at the small conjugate side. FIGS. 54(a) to 56(c) are aberration diagrams corresponding to the respective zooming states of the embodiment 25 when viewed at the small conjugate side. The spherical aberration is shown with respect to three wavelengths (WL1=610 nm, WL2=546.1 nm, WL3=470 nm), the astigmatism and the distortion are shown with respect to the e line. The respective aberrations shown in FIGS. 30 to 56 lie sufficiently in a practical range.

In the foregoing embodiments, the diaphragm AST is arranged independently behind the lens element L4 of the second lens group G2. However, an outside periphery of the small conjugate side surface of the lens element L4 may be used in place of the diaphragm AST or the diaphragm function may be realized by a construction of a lens barrel (not shown) for holding the lens element L4. Particularly, in the lens construction wherein the diaphragm is arranged right behind the fourth lens element L4 as in the foregoing embodiments 1 to 4, 7, 8, 22 to 24, these alternative diaphragm means are effective. Further in the embodiments 1 to 25, in focusing the lens system when the distance from the screen to the first lens element L1 (or L0) is changed, it is effective to move the first lens group G1 integrally along the optical axis or to move the lens groups G1, G2, and G3 integrally along the optical axis relative to the parallel plane plate L9.

Besides the construction in which the diaphragm AST has an aperture diameter thereof determined fixedly, the lens system may be constructed such that the aperture diameter is variable as already known in the conventional camera lens or the like, thereby making the effective F-number variable. The lens having the variable aperture diameter is effective, as described with reference to FIGS. 62 and 63 in relation to the fifth and sixth inventions, since it can adjust the contrast ratio and the luminance of the projected image properly in combination with a liquid crystal panel (PDLC, DSM liquid crystal, etc.) which electrically switches the state of the transmitting beam between the scattering state and the transparent state. In addition, if the aperture diameter of the diaphragm is made variable electrically by a means such as a motor as already known, the contrast ratio and the luminance can be automatically adjusted in accordance with an output of an optical sensor for detecting the brightness of the external light.

Second and Third Inventions

FIGS. 57 show a construction of a projection-type display apparatus according to the second and third inventions, FIG. 57(a) being a side view and FIG. 57(b) being a front view. In these figures, indicated at 300 is a projector including a casing 200 and a projection lens 4. A first bent mirror 41 is built in the projection lens 4. The projector 300 is accommodated in a cabinet 170 which holds a second bent mirror 160 and a translucent screen 5.

FIGS. 58 show an interior construction of the projector 300, FIG. 58(a) being a plan view and FIG. 58 being a front view. In these figure, indicated at 1 is a light source including a lamp 120., an electrode 121 of the lamp 120, a concave mirror 130, and a condenser lens 131. As the lamp 120 is used a white light source such as a metal halide lamp, a xenon lamp, and halogen lamp. In these figures is illustrated a construction in which a discharge gap is provided between a pair of electrodes 121, supposing the metal halide lamp or xenon lamp. The concave mirror 130 has a center of curvature at an emission point of the lamp 120, and is adapted to introduce the beam to the right of the lamp 120 efficiently in the drawings of FIGS. 58. A specific coating is applied to the concave mirror 130 if necessary so as to cause the infrared light to transmit therethrough, thereby reducing the degree of the radiation of heat toward the liquid crystal panel. Further, a specific coating is applied to a refracting surface of the condenser lens 131 if necessary so as to reflect the infrared rays and/or ultraviolet rays while to cause the visible light to transmit therethrough.

A filter 140 for transmitting only the visible light is arranged in an emitting direction of the light source 1. Before the filter 140 are arranged in an intersecting manner a dichroic mirror 14B for reflecting the blue light while transmitting the green and red lights and a dichroic mirror 14R for reflecting the red light while transmitting the green and blue light. Along an optical path of the reflected light from the dichroic mirror 14R are arranged mirrors 11b, 11c for reflecting all the lights and a liquid crystal panel 3R including image display unit 3r. On the other hand, along an optical path of the reflected light from the dichroic mirror 14B are arranged mirrors 11a, lid for reflecting all the lights and a liquid crystal panel 3B including image display unit 3b. Along an optical path of the transmitted light from the dichroic mirrors 14B, 14R is arranged a liquid crystal panel 3G including an image display unit 3g. A drive circuit for causing an image to be displayed on the display units 3r, 3g, and 3b of the respective liquid crystal panels 3R, 3G, and 3B is unillustrated. Enclosed by these liquid crystal panels 3R, 3G, and 3B is arranged a known dichroic prism 15 for selectively reflecting tile red and blue lights while transmitting the green light. A projection lens 4 is arranged in the emitting direction of the dichroic prism 15. The projection lens 4 is internally provided with the first bent mirror 41 and a diaphragm AST, and a detailed construction thereof is as described with respect to the first invention. FIG. 58(b) shows a case where the first bent mirror 41 is inclined at 45° as a typical example. A lens system before and after the first bent mirror 41 is unillustrated.

An operation of this embodiment will be described next. The light from the light source 1 (the light emitted from the lamp 120 and the reflected light from the concave mirror 130) is focused into a parallel beam by the condenser lens 131 to be incident upon the filter 140. The filter 140 transmits only the visible light while reflecting or absorbing the unnecessary infrared rays and ultraviolet rays. The beam transmitted through the filter 140 is incident upon the dichroic mirrors 14B, 14R. The illumination light is separated into lights of three primary colors of read (R), green (G), and blue (B) by the dichroic mirrors 14B, 14R. The blue light B has its optical path bent by the mirrors 11a, 11d so as to be projected upon the liquid crystal panel 3B. The red light R has its optical path bent by the mirrors 11b, 11c so as to be projected upon the liquid crystal panel 3R. The green light G is projected upon the liquid crystal panel 3G. In this way, since the liquid crystal panels are illuminated with the parallel beam by the presence of the condenser lens 131 (telecentric illumination system), they can be illuminated with the light of uniform chromaticity even if a spectral characteristic of the dichroic mirrors 14R, 14B has an incidence-angle dependency. Further, even in the case where a display characteristic of the liquid crystal panels 3R, 3G, and 3B have the incidence-angle dependency of the illumination light, a uniform display characteristic can be obtained in a display screen of each panel since the illumination beam is a parallel beam.

The transmitted lights modulated by the image on the liquid crystal panels are combined into a beam by the dichroic prism 15, thereby being introduced to the projection lens 4. The combined beam is bent by 90° by the first bent mirror 41, and accordingly the projected light comes out through the projection lens 4 as shown in FIG. 58(b). It is not necessary to set an inclined angle of the first bent mirror 41 at 45°. In short, the inclined angle is set at an optimum angle in designing the entire set. Those having a bent mirror inclined at an angle other than 45° belong to modifications of the present invention.

The respective image display units 3r, 3g, 3b of the liquid crystal panels 3R, 3G, 3B are equally sized, and it is required to project an image having an aspect ratio of 4 : 3 in the case of the projection-type display apparatus of the NTSC system and to project an image having an aspect ratio of 16 : 9 in the case of the projection-type display apparatus of the HDTV system in such a manner that a larger side direction becomes a horizontal direction in the projected image. However, the longer side direction of the liquid crystal display panels is converted into a lateral direction by the first bent mirror 41. Accordingly, the image display units 3r, 3g, and 3b are arranged such that the longer side direction thereof coincides with a vertical direction in the drawing of FIG. 58(b) showing a front view, i.e. a direction of a center ray 111 of the projected beam From the projection lens 4. According to the NTSC system and the HDTV system, the horizontal direction of the projected image is the longer side direction. However, there are types in which the vertical direction on the projection screen is specially the longer side direction in a projection type display for use in office automation, or the like. In this case, the image display unit should be arranged such that a direction of the shorter side of the display screen of the liquid crystal panel coincides with the vertical direction in the drawing of FIG. 58(b). A phenomenon in which the display directions of the liquid crystal panel are switched occurs similarly in the case where the first bent mirror 41 is inclined at an angle other than 45°. Thus, the direction of the side of the liquid crystal panel which is desired to be projected horizontally should be set so as to coincide with the vertical direction as in FIG. 58(b) regardless of the inclined angle of the mirror.

As described above, the diaphragm AST determines the F-number of the projection lens 4 and works to make the chief ray of the beam incident upon the projection lens 4 from the respective points of the liquid crystal panels parallel with the optical axis of the liens system. Accordingly, if the aperture diameter of the diaphragm AST is set at a large value to thereby decrease the F-number of the projection lens 4, the beams projected onto the screen 5 are increased to thereby increase the luminance. Further, if the aperture diameter of the diaphragm AST is set at a small value to thereby increase the F-number of the projection lens 4, the beams projected onto the screen 5 are decreased to thereby decrease the luminance, but the resolving power is improved since the aberrations of the projection lens 4 becomes smaller. Further, the chief ray is made into a parallel ray by the diaphragm AST as described above, which eliminates a variation in the incidence angle of the beams when they transmits through the dichroic prism 15. Thus, there can be eliminated a color shade of the projected image due to the incidence-angle dependency of the spectral characteristic of the dichroic prism 15.

Next, an operation of the entire set will be described with reference to FIGS. 57. A portion of the projector 300 before the first bent mirror 41 (casing 200) is arranged in the cabinet 170 on the left half from the center of the screen 5 in the front view (FIG. 57(b)) from the screen side. The beam bent by the first bent mirror 41 in the projection lens 4 is projected as a projected light 110 in a upward direction, and then bent toward the screen 5 by the second bent mirror 160 as shown in the side view (FIG. 57(a)), thereby forming an enlarged image on the screen 5.

The second bent mirror 160 may be rectangular, and it is sufficient for the mirror 160 to have a reflecting surface only in a range where the projected light is incident. It is effective in reducing the weight of the set if a trapezoidal mirror is used as shown by a broken line in FIG. 57(b), since an area of the mirror can be minimized. As described above, the oblique angle of the off-axis beam with respect to the optical axis can be made smaller than the oblique angle of the outgoing light from the lens with respect thereto within the lens system. Therefore, if the bent mirror is mounted in the lens, the first bent mirror 41 can be made small size. In addition, if the casing 200 is accommodated in the cabinet 170 on the left half from the center of the screen 5, a portion 171 of the cabinet 170 below the screen 5 can be made smaller. Thus, the height H of the set can be reduced easily.

In making the depth D smaller, it is effective to set an angle θ1 (see FIG. 57(a)) of the second bent mirror 160 relative to the vertical line at 45° or smaller. To this end, an angle θ2 of a bottom surface 201 of the casing 200 relative to a bottom surface 172 (coinciding with the horizontal line) of the cabinet 170 should be set such that a center line 111 of the projected beam indicated by a one-dot-one-chain line inclines more backward than vertical. Specifically, it is appropriate to set θ2≧0°. Although the center line of the projected beam incident upon the screen 5 coincides with the horizontal direction in FIG. 57 for the sake of convenience, the center line is preferably set such that an extension thereof comes to the eye position of the viewer.

In order to reduce the height H, it is required to make the casing 200 as thin as possible. To this end, in the embodiment shown in FIGS. 58, the optical system including from the light source 1 to the dichroic prism 15 is arranged on a plane perpendicular to the center ray 111 of the outgoing light of the projection lens 4 (see FIG. 58(a)) to reduce the thickness Hp of the casing 200. Indicated at 201 in FIG. 58(b) is the bottom surface of the casing 200, which orthogonally intersects with the center ray 111 of the projected light. In other words, the bottom surface 201 is arranged on a plane parallel with the plane of FIG. 58(a) showing the plan view. It can be considered to arrange the optical system by rotating the casing 200 by 90° about the optical axis of the projection lens 4, using the same optical parts as those shown in FIG. 58. However, since the width Wp of the casing 200 is greater than the thickness Hp in this case, it is disadvantageous to apply this arrangement to the construction of FIG. 57 in order to reduce the height of the entire projection-type display apparatus.

Further, the screen 5 has the aspect ratio of 4 : 3 in the apparatus of the NTSC system and of 16 : 9 in the apparatus of the HDTV system, and the longer side direction is set as the horizontal direction. As shown in FIG. 58(b), if the liquid crystal panel is arranged such that the longer side direction of the image display plane thereof becomes the vertical direction within the projector 300, the longer side direction of the projected image is conveniently converhed by the first bent mirror 41 so as to coincide with the longer side direction (horizontal direction) of the screen 5. FIGS. 57 show the example in which the casing 200 is arranged on the left half from the center of the screen 5. However, the effects of the invention are invariable even if the casing 200 is arranged on the right half from the center of the screen 5.

In order to reduce the height of the entire set under the above arrangement of the optical system in the casing, it is required to set properly the angle θ2 in FIG. 57(a) between the bottom surface 201 of the casing 200 and the bottom surface 172 of the cabinet 170 (which is caused to coincide with the horizontal direction for the sake of convenience in FIGS. 57). FIGS. 57 show the example in which the apparatus having a diagonal screen size of 40 inches (aspect ratio 3 : 4) is constructed using the projection lens according to the first invention (embodiment 1). In the illustrated example, θ1=38° and θ2=14°. Under these conditions, the height H of the set was 712 mm and the depth D thereof was 360 min. Likewise by setting θ1 =45°, θ2=0° in the screen of 40 inches, the height it was 636 mm and the depth D was 432 mm, thereby providing the arger depth while providing the smaller height than the above example. In this example, since the bottom surface of the projector coincides with the horizontal direction, the set can be easily assembled and inspected. If an increase in the depth is permitted, this construction is preferable in the light of manufacturing. Considering the condition (θ2≧0°) which reduces the depth of the set, it is desirable to satisfy the following conditional equation in order to reduce the depth and the height of the set in a well-balanced manner:

0°≦θ2≦20° (8)

If θ2 is below a lower limit value of the conditional equation (8), the depth of the set is unnecessarily large. Further, if the θ2 is in excess of an upper limit value of the conditional equation (8), following problems occurs that the height of the set is unnecessarily large, that the casing 200 shields the incident beam to the screen 5 and that the casing 200 projects forward of the set over the plane of the screen 5.

Fourth Invention

FIGS. 59 show a construction of a projection-type display apparatus according to the fourth invention, FIG. 59(a) being a plan view, FIG. 59(b) being a front view, and FIG. 59(c) being a side view. In these figures, like or corresponding reference numerals designate like or corresponding parts shown in FIGS. 57, thus no description will be given on those parts. In these figures, indicated at 41 is a bent mirror built in a projection lens 4 which is similar to the first bent mirror 41 according to the second and third inventions. FIGS. 60 show an interior construction of a projector 300, FIG. 60(a) being a plan view showing a plane where an optical system before the projection lens 4 is arranged and FIG. 60(b) being a side view. In these figures, like or corresponding reference numerals designate like or corresponding parts shown in FIGS. 58, thus no description will be given on those parts. Indicated at 51 is an optical sensor, at 52 an amplifier, at 53 a motor/lamp control circuit, and at 54 a motor.

An operation of this embodiment will be described next. The light from the light source 1 is focused into a parallel beam by a condenser lens 131 and only the visible light is transmitted through a filter 140. The beam transmitted through the filter 140 is incident upon the dichroic mirrors 14B, 14G. The illumination light is separated into lights of three primary colors of read (R), green (G), and blue (B) by the dichroic mirrors 14B, 14G. The blue light B has its optical path bent by a mirror 11a so as to be projected upon the liquid crystal panel 3B. The red light R has its optical path bent by mirrors 11b, 11c so as to be projected upon the liquid crystal panel 3R. The green light G is reflected by the dichroic mirror 14G to be projected upon the liquid crystal panel 3G. In this way, since the liquid crystal panels are illuminated with the parallel beam by the presence of the condenser lens 131 (telecentric illumination system), they can be illuminated with the light of uniform chromaticity even if a spectral characteristic of the dichroic mirrors 14B, 14G has an incidence-angle dependency. Further, even in the case where a display characteristic of the liquid crystal panels 3R, 3G, and 3B have the incidenceangle dependency of the illumination light, a uniform display characteristic can be obtained in a display screen of each panel since the illumination beam is a parallel light.

The transmitted lights modulated by the image on the liquid crystal panels are combined into a beam by the dichroic prism 15, thereby being introduced to the projection lens 4. As described in relation to the first invention, the projection lens 4 is provided internally with the bent mirror 41, and includes lens systems before and after the mirror (lens systems are unillustrated). FIG. 60(b) illustrates a case where the bent mirror 41 is inclined at 45° as a typical example. The combined beam is bent by 90° by the bent mirror 41, and accordingly a projected light 110 comes out through the projection lens 4 as shown in FIG. 60(b). The optical system before the projection lens 4 (including the light source 1 to the dichroic prism 15) is arranged on a plane parallel with a bottom surface 201 of the casing 200 (see FIG. 60 (b)) as shown in the plan view of FIG. 60(a).

Similar to the second and third inventions, a diaphragm AST determines the F-number of the projection lens 4 and works to make the chief ray of the beam incident upon the projection lens 4 from the respective points of the liquid crystal panels parallel with the optical axis of the lens system. Accordingly, if the aperture diameter of the diaphragm AST is set at a large value to thereby decrease the F-number of the projection lens 4, the beams projected onto the screen 5 are increased to thereby increase the luminance. Further, if the aperture diameter of the diaphragm AST is set at a small value to thereby increase the F-number of the projection lens 4, the beams projected onto the screen 5 are decreased to thereby decrease the luminance, but the resolving power is improved since the aberrations of the projection lens 4 are reduced. Further, the chief ray is made into a parallel ray by the diaphragm AST as described above, which eliminates a variation in the incidence angle of the beams when they transmit through the dichroic prism 15. Thus, there can be eliminated a color shade of the projected image due to the incidence-angle dependency of the spectral characteristic of the dichroic prism 15.

Next, an operation of the entire set will be described with reference to FIGS. 59. A portion of the projector 300 before the bent mirror 41 (casing 200) is arranged in the cabinet 170 on the left half from the center of the screen 5 in the plan view (FIG. 59(a)) from the above, and the bottom surface 201 of the casing 200 is arranged substantially in parallel with the screen 5. The beam bent by the bent mirror 41 in the projection lens 4 is projected as the projected light 110 on the screen 5 to form an enlarged image thereon. In this embodiment, a center ray 111 of the beam from the projection lens 4 advances in the horizontal direction, and incident upon the screen 5 substantially perpendicularly to the screen 5.

As described in relation to the first invention, if the bent mirror 41 is arranged in the lens, an oblique angle of an off-axis beam relative to the optical axis can be made smaller than an oblique angle of an outgoing beam relative thereto. Accordingly, the size of the mirror can be made smaller compared to a case where the mirror is arranged outside the projection lens 4. By accommodating the casing 200 in the left half of the cabinet 170 from the center of the screen 5, the front dimensions (W, H in FIG. 59(b)) of the cabinet 170 can be made substantially equal to the dimensions of the screen 5. In addition, using the wideangle projection lens 4 as shown in the first invention, the depth of the cabinet 170 (D in FIG. 59(c)) can be reduced.

As described above, the bottom surface 201 of the casing 200 is arranged substantially in parallel with the screen 5 so that the center ray 111 of the projected light 110 advances in the horizontal direction, thereby reducing the depth D while minimizing the front dimensions W, H of the apparatus. It can be considered in FIG. 59(b) to arrange the casing 200 rotated by 90° about the center ray 111 using the same projector as in FIGS. 60. It goes without saying that such an arrangement of the projector belongs to modifications of the present invention. FIGS. 59 show the construction in which the casing 200 is arranged on the left half from the center of the screen 5. However, there is no problem in arranging the casing 200 on the right half.

A modification of the fourth invention described above will be described with reference to FIG. 61. FIG. 61 shows an exemplary construction of a multi-vision projector in which a plurality of projectors 300 shown in FIGS. 60 in rows and columns. In this figure, indicated at 300a to 300d are projectors similar to the one shown in FIGS. 60. The illustrated example shows an array of 3 (row)×4 (column), but projectors 300e to 300l and projection lenses 4e to 4l in the two lower rows are unillustrated for the sake of simplicity. Indicated at 5a to 5l are screens on which the projected lights coming from the projection lenses 4a to 4l are incident to form unit projected images. The respective projectors are so arranged that the adjoining unit images are arrayed closely on the screens 5a to 5l. Bottom surface of casings of the respective unit projectors 300a to 300l are arranged in parallel with the screens 5a to 5l similar to the case shown in FIG. 59. Indicated at 170a to 170l are cabinets for accommodating the projectors 300a to 300l and holding the screens 5a to 5l.

The screens 5a to 5l may be arrayed in rows and columns separately for each unit screen, or may constitute an integral screen. Alternatively, the screens 5a to 5l may partially constitute semi-integral screens corresponding to collections of unit screens of, e.g. 2×2. For example, {5a, 5b, 5e, 5f}, {5c, 5d, 5g, 5h} may constitute the semi-integral screens, and [5i, 5j}, {5k, 5l} may constitute other semi-integral screens. Further, as the cabinets 170a to 170l, separate box-like cabinets may be arrayed in rows and columns. Alternatively, it may be appropriate to form several of these cabinets into a box-like semi-integral cabinet and to array these semi-integral cabinets, or to form all of these into a single box-like integral cabinet.

As described above, the multi-vision projector shown in FIG. 61 is constructed by arraying a plurality of projection-type display apparatuses shown in FIG. 59 having the front dimensions W, H substantially equal to the dimensions of the screen 5 and the small depth D. Accordingly, the apparatus is allowed to have a small depth D in which apparatus adjoining unit images are arranged with little spacing. Though the array of the unit screens is 3 (row)×4 (column) in the example of FIG. 61, the array is not limited to this. Since this embodiment is a multi-vision construction, there can be easily realized the apparatus having an extremely large screen, a high resolution, and a high luminance.

Further, by manufacturing the projection lenses 4a to 4l into zoom lenses in the construction of FIG. 61, the projection magnifications carl be easily adjusted to the same value even if there is any difference in the projection magnification between the respective screens, thereby minimizing the discontinuity of the images between the unit screens.

The dichroic prism 15 in the foregoing second, third, and fourth inventions may be formed of glass material. However, the apparatus can be manufactured light weighted and inexpensively by using the plastic materials (PMMA, PC, ZEONEX, OZ1000, ARTON, APO, etc.) as described in the embodiment 13 of the first invention.

Further, if the projection lens 4 is manufactured as the zoom lens described in the embodiment 25 of the first invention, the dimensions of the projected image can be made adjustable. As already known, a lenticular screen having a periodic structure in the horizontal direction is used as the screen 5, and the liquid crystal panel 3 has a periodic structure caused by the matrix-like electrodes. Accordingly, periodic structures can be observed in the enlarged projected image in the horizontal and vertical directions. Out of these periodic structures, it is known that the vertically striped moire is generated resulting from the interference of the horizontal period structures of the screen 5 and the liquid crystal projected image, thereby preventing viewers from viewing the images clearly. It is already known that the visibility is lowered by optimizing a ratio of the horizontal period of the screen 5 to that of the liquid crystal panel, to thereby minimize the bad effect of the vertically striped moire to the image viewing. Thus, if the projection lens 4 is manufactured as a zoom lens and the projection magnification is adjusted, the moire which is superimposed on the projected image can be adjusted to the degree that it becomes hardest to see.

Fifth Invention

Liquid crystal materials based on various principles of operation including conventionally known TN (Twisted Nematic) liquid crystal, PDLC (Polymer Dispersed Liquid Crystal), and DSM (Dynamic Scattering Mode) liquid crystal are applied in the liquid crystal panel for use in the projection-type display apparatus. The PDLC and the DSM liquid crystal are known as those whose state are switched between the scattering state and the transparent state according to a voltage applied to pixels. Accordingly, in the case where the projected image is formed by means of the projection lens 4, the projected beam is shielded by the diaphragm AST provided in the projection lens 4 of FIG. 58 when the liquid crystal panel is in the scattering state. Thus, the projected image is obliged to have a dark level. On the other hand, when the liquid crystal panel is in the transparent state, the projected image has a bright level since the parallel beam having transmitted through the panel is projected through the projection lens 4 almost without any loss.

The constructional feature of the projection-type display apparatus according to the fifth invention is to make the aperture diameter of the diaphragm AST variable in the projection-type display apparatus using the PDLC and the DSM liquid crystal. Novel effects brought about by this construction will be described with reference to FIGS. 62(a)-62(d) showing diagrammatically the principle of the fifth invention. In FIGS. 62(a)-62(d), indicated at EP is an entrance pupil of the projection lens 4 which is variable according to the aperture diameter of the diaphragm AST, and a single liquid crystal panel 3 is shown for the sake of convenience. FIG. 62(b) and FIG. 62(d) show a case where the aperture diameter of the diaphragm AST is increased, while FIG. 62(a) and FIG. 62(c) show a case where the aperture diameter of the diaphragm AST is decreased.

FIGS. 62(a)-62(d) are explained supposing the liquid crystal as the type whose state is switched between the transparent state and the scattering state (PDLC, DSM, etc.). In the case where the liquid crystal panel 3 is in the transparent state, the illumination light Iin becomes a parallel transmitted light It, and further becomes a projected beam through the entrance pupil EP without loss. On the other hand, in the case where the liquid crystal panel 3 is in the scattering state, the transmitted light becomes a diffused beam Id. Thus, the beam within a recipient angle θ0 of the entrance pupil EP becomes the projected beam through the entrance pupil EP, but the beam outside the recipient angle θ0 indicated by oblique lines is not projected. It can be seen that if the aperture diameter of the entrance pupil EP is changed by adjusting the aperture diameter of the diaphragm AST, the intensity of the projected beam when the liquid crystal panel 3 is the scattering state (dark level) is changed to thereby change the contrast ratio (a ratio of the bright level. to the dark level of the projected image).

Since the illumination light (Iin in FIG. 62(a)-62(d)) incident upon the liquid crystal panel is in actuality not a perfectly parallel light, the luminance of the projected image when the liquid crystal panel is in the transparent state (bright level) is also changed by changing the aperture diameter of the diaphragm AST. More specifically, by constructing the diaphragm AST in the projection lens of the first invention so that the aperture diameter D thereof (effective F-number of the projection lens) can be made variable by means of a known diaphragm blade 42 or the like, the contrast ratio and the luminance level can be adjusted optimally according to an optical characteristic of the liquid crystal panel and the viewer's preference. Even if the liquid crystal panel is of the type whose state is not switched between the transparent state and the scattering state such as the TN liquid crystal, the luminance of the projected image (bright level) can be mainly changed since the illumination light is not perfectly parallel as described above. Thus, the invention is of course applicable to such a type of liquid crystal panel. Further, since the aberrations of the projection lens are reduced if the aperture diameter of the diaphragm AST is decreased, the projected image is formed with a high resolution.

The embodiment of the fifth invention is described using the construction of the projector according to the second and third inventions shown in FIG. 58 as an exemplary construction. However, according to the fifth invention, an essential element of the second and third inventions, namely the first bent mirror 41, is not necessarily provided in the projection lens 4. The projection lens may be of the type including a known directly operable barrel structure. Further, the projection system is not limited to the rear system, but may be the front system using a reflection type screen.

Sixth Invention

In a projection-type display apparatus according to the sixth invention, the aperture diameter of the diaphragm AST and/or a drive power of the lamp are controlled according to an output of an optical sensor for detecting the brightness of the external light in a place where the apparatus is installed, thereby automatically adjusting the luminanee and/or contrast ratio of the projected image according to the state of the external light. The basic concept to adjust the aperture diameter of the diaphragm AST is sin, ilar to that of the fifth invention. FIG. 63(b) and FIG. 63(d) show a case where the aperture diameter of the diaphragm AST is increased, while FIG. 63(a) and FIG. 63(c) shows a case where the aperture diameter of the diaphragm AST is decreased. According to the sixth invention, the aperture diameter D is made variable by driving a diaphragm blade 42 by means of a motor 54, thereby optimally adjusting the contrast ratio and the luminance level of the projected image.

An operation of a projector according to the sixth invention will be described with reference to FIGS. 60 which are referred to explain the fourth invention. An optical sensor 51 detects the external light of the environment where the projection-type display apparatus is placed, and outputs an electric signal according to the brightness of the external light. An amplifier 52 amplifies the output of the optical sensor 51. Subsequently, a drive signal to be applied to the motor 54 and a drive power signal to be applied to a lamp 120 are generated in a motor/lamp control circuit 53 in accordance with the amplified output. The motor 54 adjusts the aperture diameter of the diaphragm AST according to an output of the control circuit 53. Specifically, the aperture diameter of the diaphragm AST is increased when the external light is bright while being decreased as the external light becomes darker. In the case where the liquid crystal material of the type whose state is switched between the transparent state and the scattering state such as the PDLC and the DSM liquid crystal is used as liquid crystal for the liquid crystal panels 3R, 3G, 3B, the projected image can be formed as follows by controlling the diaphragm AST as described above. When the external light is bright, the projected image of high luminance is formed at the reduced contrast ratio and resolution. On the other hand, when the external light is dark, the projected image of high resolution can be formed at the low luminance and high contrast ratio. In the case where the liquid crystal material of the type whose state is not switched between the transparent state and the scattering state such as TN liquid crystal is used as liquid crystal for the liquid crystal panels 3R, 3G, 3B, the luminance and resolution of the projected image can be mainly controlled by changing the aperture diameter of the diaphragm AST.

In the apparatus shown in FIGS. 60, not only the aperture diameter of the diaphragm AST but also the drive power of the lamp 120 is controlled according to the output of the control circuit 53. More specifically, the drive power of the lamp 120 is increased to thereby form the projected image of high luminance when the external light is bright, while being decreased to thereby form the projected image of low luminance when the external light is dark. It may be appropriate to execute both the diaphragm control and the lamp drive power control, or it may be sufficient to execute only either one of these.

Next, description is given on the mounting of the optical sensor 51 with reference to FIGS. 59. In these figures, the optical sensor 51 is arranged at an end of the screen 5 with a light receiving surface thereof faced outward of the cabinet 170. With this arrangement, the optical sensor 51 is allowed to detect the brightness of the external light incident upon the surface of the screen 5, and the processing after the amplifier 52 in FIG. 60 can be carried out.

The embodiment of the sixth invention is described using the construction of the projector according to the fourth invention shown in FIGS. 60 as an exemplary construction. However, according to the sixth invention, an essential element of the fourth invention, namely the bent mirror 41, is not necessarily provided in the projection lens 4. The projection lens may be of the type including a known directly operable barrel structure not having a mirror. Further, the projection system is not limited to the rear system, but may be the front system using a reflection type screen. In the case of the front projection system, it is preferable either to dispose the optical sensor 51 on the reflection type screen or to dispose it at a part of or in the vicinity of the casing 200.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims. 

What is claimed is:
 1. A retro-focus type lens comprising from a large conjugate side:a first lens group having a negative refracting power; a second lens group having a positive refracting power which is spaced away from the first lens group at an air interval D1; and a third lens group having a positive refracting power; wherein the following conditions are satisfied when focal lengths of the first lens group, the second lens group, the third lens group, and a total lens system are assumed respectively as f1, f2, f3, and f: 1.8<D1/f<3.0 1.1<f2/f3<1.6 1.5<|f1|/f<2.3.
 2. A retro-focus type lens according to claim 1, further comprising mirror means which is inserted between the first lens group and the second lens group and is adapted to bend an optical path.
 3. A retro-focus type lens according to claim 1, further comprising diaphragm means arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side.
 4. A retro-focus type lens according to claim 1, further comprising diaphragm means arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side, wherein an aperture diameter of the diaphragm means is adjustable.
 5. A retro-focus type lens according to claim 1, further comprising:diaphragm means arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side; and motor means for causing an aperture diameter of the diaphragm means to vary.
 6. A retro-focus type lens according to claim 1, wherein a lens element on the small conjugate side is an aspherical lens out of those constituting the third lens group.
 7. A retro-focus type lens according to claim 1, wherein a lens element on the small conjugate side is an aspherical lens out of those constituting the third lens group, and the following condition is satisfied when focal lengths of the aspherical lens and the total lens system are assumed respectively as f4 and f:|f4|/f>10.
 8. A retro-focus type lens according to claim 1, wherein a lens element on the large conjugate side is an aspherical lens out of those constituting the first lens group.
 9. A retro-focus type lens according to claim 1, wherein a lens element on the large conjugate side is an aspherical lens out of those constituting the first lens group, and the following condition is satisfied when focal lengths of the aspherical lens and the total lens system are assumed respectively as f5 and f:|f5|/f>100.
 10. A retro-focus type lens according to claim 1, wherein a lens element on the small conjugate side out of those constituting the first lens group and a lens element constituting the second lens group are both aspherical lenses.
 11. A retro-focus type lens according to claim 1, wherein an aberration correction is carried out in a state where a parallel plane plate formed of plastic material in not less than 90% of its thickness is inserted between the third lens group and an image surface on a small conjugate side.
 12. A retro-focus type lens according to claim 1, further comprising diaphragm means arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side,wherein the second lens group is moved along an optical axis to change the focal length of the total lens system, and the first lens group, the second lens group, and the third lens group are integrally moved along the optical axis to change a projection magnification while compensating for a shift of an image surface.
 13. A retro-focus type lens according to claim 1, further comprising diaphragm means arranged in the vicinity of the second lens group and a focal point of the third lens group on the large conjugate side,wherein the second lens group is moved along an optical axis to change the focal length of the total lens system, and the projection magnification is changed while compensating for a shift of an image surface by integral movement of the first lens group, the second lens group, and the third lens group along the optical axis in combination with movement of the first lens group along the optical axis. 